Retire AFS_MOUNT_AFS

Retire AFS_MOUNT_AFS

Currently, the AFS_MOUNT_AFS #define is used to mean two completely
different things:

- The string "afs", corresponding to the first argument to mount(2) on
  many platforms and some related calls inside libafs (e.g.
  getnewvnode() on FBSD).

- An integer identifying the AFS filesystem (e.g. gfsadd() on AIX).

Depending on the platform and the build context (UKERNEL vs KERNEL),
AFS_MOUNT_AFS gets defined to one of those two things. This is very
confusing, and has led to mistakes in the past, such as those fixed in
commit 446457a1 (afs: Set AFS_VFSFSID to a numerical value).

To avoid such confusion, get rid of AFS_MOUNT_AFS completely, and
replace it with two new symbols:

- AFS_MOUNT_STR, the string "afs".

- AFS_FSNO, the integer given to gfsadd() et al.

When AFS_MOUNT_AFS is split this way, AFS_MOUNT_STR then is always
defined to the same value, so remove it from the param.h files for our
platforms. Instead, define it in afs.h for libafs use, and in
afsd_kernel.c (the only place outside of src/afs that uses it).

Also remove the logic for conditionally defining MOUNT_AFS from the
param.h files, moving the logic to the same locations as
AFS_MOUNT_STR.

Note that this commit removes the numeric definition for AFS_MOUNT_AFS
in param.sgi_65.h (aka AFS_FSNO). We never actually used this value,
since AFS_FSNO is not used on IRIX; instead, we tend to use the
'afs_fstype' global instead of a constant number.

Change-Id: I6cbf051dc938cd1c456cbe236c0afe99a3c3dd87
Reviewed-on: https://gerrit.openafs.org/14323
Reviewed-by: Cheyenne Wills <cwills@sinenomine.net>
Reviewed-by: Michael Meffie <mmeffie@sinenomine.net>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>