Stop using sig_t

Stop using sig_t

acinclude.m4 was adding to afsconfig.h some code to define sig_t as the
type of a signal handler.  This definition was in terms of RETSIGTYPE,
which is deprecated by Autoconf.  It also adds an unnecessary abstraction
for only two places in the AFS source code where it was used.  Remove it
and change those two spots to use the standard C declaration of the
function pointer.

Reviewed-on: http://gerrit.openafs.org/335
Tested-by: Simon Wilkinson <sxw@inf.ed.ac.uk>
Reviewed-by: Simon Wilkinson <sxw@inf.ed.ac.uk>
Reviewed-by: Derrick Brashear <shadow@dementia.org>