Do not require AFS_SYSCALL

Do not require AFS_SYSCALL

Various parts of the code make use of AFS_SYSCALL in order to
communicate with the libafs kernel module. Even though most modern
platforms do not use an actual syscall anymore (instead using an
ioctl-based method or similar to emulate the traditional AFS syscall),
some code paths rely on AFS_SYSCALL as a fallback, or just use
AFS_SYSCALL because they were never updated to use the newer methods.

Even platforms that do not use the traditional AFS syscall still
define the AFS_SYSCALL number, in case someone still uses it for
something. However, some platforms do not have an AFS syscall number;
there is no "slot" allocated to us, so we cannot safely issue any
syscall.

For those platforms, we must not reference AFS_SYSCALL at all, or we
will fail to build. So, get rid of these references to AFS_SYSCALL if
it is not defined. In some places, we can just avoid the relevant code
making the syscall. In a few other places, we just pretend like the
libafs kernel module was not loaded and yield an ENOSYS error, to make
the code simpler.

Change-Id: I38e033caf7149c2b1b567f9877221ca8551db2ea
Reviewed-on: https://gerrit.openafs.org/11937
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Ian Wienand <iwienand@redhat.com>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>