Shell: Properly set and restore termios settings.

Previously, we did not properly restore termios settings
after running built-in commands. This has been fixed
by ensuring that we only change the termios settings
when we are forking a child process.

Shell/main.cpp

@@ -634,7 +634,6 @@ static int run_command(const String& cmd)
 
  struct termios trm;
  tcgetattr(0, &trm);
- tcsetattr(0, TCSANOW, &g.default_termios);
 
  struct SpawnedProcess {
  String name;
@@ -730,6 +729,7 @@ static int run_command(const String& cmd)
  if (!child) {
  setpgid(0, 0);
  tcsetpgrp(0, getpid());
+ tcsetattr(0, TCSANOW, &g.default_termios);
  for (auto& rewiring : subcommand.rewirings) {
 #ifdef SH_DEBUG
  dbgprintf("in %s<%d>, dup2(%d, %d)\n", argv[0], getpid(), rewiring.rewire_fd, rewiring.fd);