Make UKERNEL's panic() a real function, not a macro

In particular, a varargs function, so that osi_Panic() can always
call panic() as varargs.

Change-Id: I2ceee737c0025c146a8cd17b16d6c159e2bbf9d0
Reviewed-on: http://gerrit.openafs.org/9877
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Simon Wilkinson <simonxwilkinson@gmail.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>