LINUX: Define printf/uprintf as variadic macros

LINUX: Define printf/uprintf as variadic macros

Instead of defining the string 'printf' itself, make printf (and
uprintf) variadic macros. This avoids renaming printf to printk for
things like '__attribute__((format(printf,X,Y)))'.

Note that this is Linux-specific; compilers on other platforms may not
support variadic macros.

This avoids many warnings in the Linux kernel module build if we
include Linux headers after AFS headers.

Reviewed-on: http://gerrit.openafs.org/8150
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
Reviewed-by: Marc Dionne <marc.c.dionne@gmail.com>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
(cherry picked from commit 179096d9b2c461f02236bbf670b46597ff2d4c3c)

Change-Id: I5c1c80cb5bd6996b0329969e16f9359fa1dcbc91
Reviewed-on: https://gerrit.openafs.org/12365
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Michael Meffie <mmeffie@sinenomine.net>
Reviewed-by: Mark Vitale <mvitale@sinenomine.net>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>