Restore compatibility with BSD editline and older GNU readline versions

input: block all signals in atexit(3) handler

This fixes a potential race condition where the execution of the cleanup
function form atexit(3) would be interrupted by a signal, causing
cleanup to be called again.

input: update llen after realloc(3)

This reduces the amount of reallocs performed by getline(3).

Unconditionally set signal handler

The signal handler doesn't do anything if neither -h nor -c where given.
Without this change the temporary file weren't removed if -c was given
but -h wasn't.

Don't leave sa_flags uninitialized

The sigaction(3) implementation must read this struct member to figure
out what kind of handler function is being used (sa_sigaction vs.
sa_handler).

input.1: Clarify that received input is written to stdout