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.

Change-Id: I2b8ed67f25e93f559535e40680ecab6fe7579e9c
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>