afsd: Tidy up system calls

afsd: Tidy up system calls

Tidy up the way that we do system calls from afsd, by making
afsd_syscall a va_arg function, using a structure to pass system call
information around, and simplifying the #ifdef ladder that converts our
platform independent system calls into something platform specific.

This fixes all of the warnings in afsd which required the -Wno-error
option, the only warnings remaining are related to daemon being
deprecated on Darwin.

Change-Id: I357c7d175b5327007d50d213c6181881c842050e
Reviewed-on: http://gerrit.openafs.org/7188
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Tested-by: Derrick Brashear <shadow@dementix.org>
Reviewed-by: Derrick Brashear <shadow@dementix.org>