Add printf format checks to afs_com_err()

Add printf format checks to afs_com_err()

Add gcc printf format checks to the afs_com_err() functions

Deal with the fallout, in particular change callers which pass
an empty format string to pass NULL instead - the com_err functions
already permit this alternate use.

There's a couple of real bugs here - in one case, we attempt to
print a NULL pointer, rather than a security index, and in the other
we supply a NULL format string, rather than the string we meant to
print.

Change-Id: Icd48f92a4447d4af3dba9a4caa2ff73c1657ad47
Reviewed-on: http://gerrit.openafs.org/794
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>