DARWIN: Replace afs_osi_cred by allocated ucred

DARWIN: Replace afs_osi_cred by allocated ucred

Building the current version of OpenAFS on macOS Monterey arm64 results
in the following error:

 error: globals with authenticated null values are currently unsupported
 afs_ucred_t afs_osi_cred;

To avoid this problem, replace the global in question by a pointer to a
dynamically allocated afs_osi_cred (afs_osi_credp). Ideally we would
create a new credential with kauth_cred_create(), but this function is
not well documented and using it results in crashes. Moreover, if the
kauth_cred_dup() function were still available (private since XNU
1456.1.26 - macOS 10.6), we could also duplicate the current user's
credential during startup (like commit f40f466c7f did for BSD).

Change-Id: Ied2df7590e4e671ce2643669c83ee0e4b2a97cd1
Reviewed-on: https://gerrit.openafs.org/14922
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Tested-by: BuildBot <buildbot@rampaginggeek.com>