checkincludes.pl: add option to remove duplicates in place

checkincludes.pl is more useful if it actually removed the lines.  This
adds support for that with -r.

[akpm@linux-foundation.org: improve usage message]
Signed-off-by: Luis R. Rodriguez <lrodriguez@atheros.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>