checkpatch: Add warning for p0-patches

Some people work internally with -p0-patches which has the danger that one
forgets to convert them to -p1 before mainlining.  Bitten myself and seen
p0-patches in mailing lists occasionally, this patch adds a warning to
checkpatch.pl in case a patch is -p0.  If you really want, you can fool
this check to generate false positives, this is why it just spits a
warning.  Making the check 100% proof is trickier than it looks, so let's
start with a version which catches the cases of real use.

[apw@canonical.com: update message language, handle null prefix, add tests]
Signed-off-by: Wolfram Sang <w.sang@pengutronix.de>
Signed-off-by: Andy Whitcroft <apw@canonical.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>