Postmoderize use of AC_CHECK_TYPE

Postmoderize use of AC_CHECK_TYPE

Use AC_TYPE_SSIZE_T to handle the ssize_t check (introduced in Autconf
2.60).  Use AC_CHECK_TYPE, not AC_CHECK_TYPES, to check for sig_atomic_t
and socklen_t, and define them in afsconfig.h if they're not present on
this platform.  Do not define them in stds.h, since stds.h is an installed
header file and cannot rely on Autoconf defines.

Change-Id: I80e1a00efc84420002a95d6cafc25ea70befa49d
Reviewed-on: http://gerrit.openafs.org/2018
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>