mrsh/frontend d---------
Drop mrsh_limit.h

We don't rely on PATH_MAX anymore.

Closes: https://github.com/emersion/mrsh/issues/167
frontend: don't rely on PATH_MAX for readline history

References: https://github.com/emersion/mrsh/issues/167

According to POSIX, PATH_MAX may or may not be defined.
Let's define it in case it is not set.

Fix https://github.com/emersion/mrsh/issues/143
frontend: PATH_MAX includes the terminating null char
Add stub for rl_replace_line

Closes: https://github.com/emersion/mrsh/issues/119
frontend/readline: fix stupid GCC warnings
frontend/readline: fix undefined rl_replace_line with editline

editline doesn't define rl_replace_line. I haven't found a good way to make
SIGINT handling work with editline, let's just disable it for now.

Fixes: 763feb2ee5dd ("readline: reset on SIGINT")
readline: reset on SIGINT
Change fprintf(stderr, …, strerror(errno)) to perror
meson: enable more compiler warnings
Fix error handling in basic frontend
Add editline support
Move frontend_* -> frontend/*