Use 'void' instead of 'RETSIGTYPE'. Use 'sighandler_t' too.

Use 'void' instead of 'RETSIGTYPE'.  Use 'sighandler_t' too.

C89 and POSIX.1-2001 define signal(2) as returning a pointer to a
function returning 'void'.  K&R C signal(2) signature is obsolete.
Use 'void' directly.

Also, instead of writing the function pointer type explicitly, use
POSIX's 'sighandler_t'.

Signed-off-by: Alejandro Colomar <alx.manpages@gmail.com>