Include unistd.h as it is the standard location for getopt().

OK millert@
missing unsigned char casts areound tolower()
ok nicm
Handle empty current directory more gracefully.
With -k, kill window after using it to work out -c path. Reported by
jmacristovao at gmail dot com.
Replace ## by # in format.
Add comments to ACS table matching terminfo(5).
Remove unnecessary calls to va_start/va_end, from Tiago Cunha.
Simplify args_set, from Tiago Cunha.
Style and comment fixes from Tiago Cunha.
Three small changes from Tiago Cunha:

- Check for truncation when copying path.
- Don't need to use a temporary buffer in screen_set_title.
- Include strerror in output when connecting to server fails.
Allow attach-session -t to accept a window and pane to select them on
attach. Based on a diff from J Raynor.
Similar to attach-session, make switch-client -t accept a window and
pane. From Johannes Jakobsson.
Fix a memory/fd leak reported by Tiago Cunha.
Couple of fixes from cppcheck via Tiago Cunha.
Do not attempt to read .tmux.conf if we can't figure out a home
directory, from Tiago Cunha.
Merge branch 'obsd-master'
Only exit copy mode at the bottom if no selection in progress, from
Benoit Pierre.
Do not permit periods in session names (colons are already banned). From
J Raynor.
+ to TODO.
Add AC_PROG_MKDIR_P.