Make all signal handler functions async-signal-safe

Make all signal handler functions async-signal-safe
by deleting the redundant "killersig" struct member
and using the existing sig_atomic_t cl_sigterm variable instead.
While here, garbage collect the h_hup() signal handler
which is essentially identical to h_term().
This also gets rid of the last #define & #undef in cl_main.c.

OK martijn@, and also tested by Tim <trondd at kagu hyphen tsuchi dot com>.