Rework preexec hooks to be more centralized

It seems we are constantly adding new features that need new preexec
hooks. Rather than continue to bloat prompt_on() and
__lp_disable_hooks(), break everything out into a __lp_preexec()
function that can be expanded easily.

Also note that I found a bug: removing the DEBUG trap does not work, as
the DEBUG trap can be set inside a function and only apply to that
function. So removing it will unset the trap for that function only.
Likely there is a way around this with setting the shell option that
makes functions inherit the DEBUG trap of the global scope, but it needs
a lot of testing, and this has likely been broken from the start.

liquidprompt

@@ -830,6 +830,12 @@ lp_activate() {
 
  unset -f _lp_require_tool
 
+ _LP_ENABLE_PREEXEC=$((
+ LP_ENABLE_RUNTIME \
+ || LP_ENABLE_RUNTIME_BELL \
+ || LP_ENABLE_TITLE_COMMAND \
+ ))
+
  # LP_ENABLE_RUBY_VENV depends either from rvm or rbenv. Thus we cannot
  # directly use _lp_require_tool for it.
  # Also, to avoid to check twice which is the current ruby virtual env
@@ -4843,6 +4849,15 @@ __lp_set_prompt() {
  fi
 }
 
+__lp_preexec() {
+ if (( LP_ENABLE_RUNTIME || LP_ENABLE_RUNTIME_BELL )); then
+ __lp_runtime_before
+ fi
+ if (( LP_ENABLE_TITLE_COMMAND )); then
+ __lp_print_title_command
+ fi
+}
+
 __lp_before_command() {
  # For debugging
  #printf 'XXX %s\n' "$BASH_COMMAND"
@@ -4851,14 +4866,7 @@ __lp_before_command() {
  # execute the hooks.
  if (( _LP_AT_PROMPT )); then
  _LP_AT_PROMPT=0
-
- if (( LP_ENABLE_RUNTIME || LP_ENABLE_RUNTIME_BELL )); then
- __lp_runtime_before
- fi
-
- if (( LP_ENABLE_TITLE_COMMAND )); then
- __lp_print_title_command
- fi
+ __lp_preexec
  fi
 
  # If this is the last thing before prompt is being drawn, the command is done,
@@ -4928,7 +4936,7 @@ prompt_on() {
  PROMPT_COMMAND="$set_prompt_command"
  fi
 
- if (( LP_ENABLE_RUNTIME || LP_ENABLE_RUNTIME_BELL || LP_ENABLE_TITLE_COMMAND )); then
+ if (( ${_LP_ENABLE_PREEXEC-0} )); then
  _LP_AT_PROMPT=0
  _LP_RUNTIME_LAST_SECONDS=$SECONDS
  # __lp_before_command gets called just before bash executes a command,
@@ -4955,17 +4963,11 @@ prompt_on() {
  precmd_functions+=(__lp_set_prompt)
  fi
  fi
- if (( LP_ENABLE_RUNTIME || LP_ENABLE_RUNTIME_BELL )); then
+ if (( ${_LP_ENABLE_PREEXEC-0} )); then
  _LP_RUNTIME_LAST_SECONDS=$SECONDS
  # It's less bad to have this be duped than __lp_set_prompt, but let's be sure
- if ! ( __lp_array_contains __lp_runtime_before ${preexec_functions[@]+"${preexec_functions[@]}"} ); then
- preexec_functions+=(__lp_runtime_before)
- fi
- fi
- if (( LP_ENABLE_TITLE_COMMAND )); then
- # It's less bad to have this be duped than __lp_set_prompt, but let's be sure
- if ! ( __lp_array_contains __lp_print_title_command ${preexec_functions[@]+"${preexec_functions[@]}"} ); then
- preexec_functions+=(__lp_print_title_command)
+ if ! ( __lp_array_contains __lp_preexec ${preexec_functions[@]+"${preexec_functions[@]}"} ); then
+ preexec_functions+=(__lp_preexec)
  fi
  fi
  fi
@@ -4991,13 +4993,10 @@ prompt_on() {
 
  add-zsh-hook precmd __lp_set_prompt
 
- if (( LP_ENABLE_RUNTIME || LP_ENABLE_RUNTIME_BELL )); then
+ if (( ${_LP_ENABLE_PREEXEC-0} )); then
  _LP_RUNTIME_LAST_SECONDS=$SECONDS
  declare -gi _LP_RUNTIME_SECONDS
- add-zsh-hook preexec __lp_runtime_before
- fi
- if (( LP_ENABLE_TITLE_COMMAND )); then
- add-zsh-hook preexec __lp_print_title_command
+ add-zsh-hook preexec __lp_preexec
  fi
  fi
 }
@@ -5015,7 +5014,7 @@ __lp_disable_hooks() {
 
  for i in ${preexec_functions[@]+"${!preexec_functions[@]}"}; do
  local value="${preexec_functions[i]}"
- if [[ $value == "__lp_runtime_before" || $value == "__lp_print_title_command" ]]; then
+ if [[ $value == "__lp_preexec" ]]; then
  unset 'preexec_functions[i]'
  fi
  done
@@ -5033,21 +5032,21 @@ __lp_disable_hooks() {
  # shellcheck disable=SC2178
  PROMPT_COMMAND="${LP_OLD_PROMPT_COMMAND-}"
  unset LP_OLD_PROMPT_COMMAND
+ else
+ unset PROMPT_COMMAND
  fi
  fi
 
  # Disable the DEBUG trap used by the RUNTIME or TITLE_COMMAND features
- if (( ${LP_ENABLE_RUNTIME-0} || ${LP_ENABLE_RUNTIME_BELL-0} || ${LP_ENABLE_TITLE_COMMAND-0} )); then
+ # TODO: Fix this: Bash will unset the function's DEBUG trap, not the global one.
+ if (( ${_LP_ENABLE_PREEXEC-0} )); then
  trap - DEBUG
  fi
  fi
  else # zsh
- # Disable previous hooks as options that set them
- # may have changed
  {
  add-zsh-hook -d precmd __lp_set_prompt
- add-zsh-hook -d preexec __lp_runtime_before
- add-zsh-hook -d preexec __lp_print_title_command
+ add-zsh-hook -d preexec __lp_preexec
  } >/dev/null
  fi
 }