LINUX: Always crref after _settok_setParentPag

LINUX: Always crref after _settok_setParentPag

Commit b61eac78 (Linux: setpag() may replace credentials) changed
PSetTokens2 to call crref() after _settok_setParentPag(), since
changing the parent PAG may change our credentials structure. But that
commit did not update the old pioctl PSetTokens, so -setpag
functionality remained broken on Linux for utilities that called the
old pioctl ('klog' is one such utility).

To fix this, we could copy the same code from PSetTokens2 into
PSetTokens. But instead just move this code into _settok_setParentPag
itself, to avoid code duplication. This commit also refactors
_settok_setParentPag a little to make the platform-specific ifdefs a
little easier to read through.

Change-Id: I65a165ebb1d823e690926de31b28a7728d2561b9
Reviewed-on: https://gerrit.openafs.org/14147
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Yadavendra Yadav <yadayada@in.ibm.com>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>