scripts/patch-kernel: use EXTRAVERSION

scripts/patch-kernel: use EXTRAVERSION

Update 'scripts/patch-kernel' to support EXTRAVERSION.
Update usage message text.
Fix some whitespace.
Handle command line arg3 (stop-version) more carefully.
No changes to -ac patch updates.

EXTRAVERSION handling:
any leading '.' and any trailing modifier (beginning with any
punctuation, like "-ac" or "_kexec" or "+mm") are stripped,
trying to get down to just a number.
Then 'patch-kernel' increments EXTRAVERSION as long as it can
apply "patch-V.P.S.X*".  When that file isn't found, it resets
EXTRAVERSION to "" and increments SUBLEVEL (as before this patch).

Works for me.

Signed-off-by: Randy Dunlap <rddunlap@osdl.org>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>