Enhance "make xconfig".

Signed-off-by: Shlomi Fish <shlomif@shlomifish.org>

This is a modified version of this patch:

http://www.shlomifish.org/open-source/projects/linux-kernel/xconfig-search/

with the main changes being that most of the warnings and errors
reported by checkpatch.pl were fixed.

This patch adds substring, keyword search and regular expression search
to the "make xconfig" Edit -> Find dialog in the kernel - all with
case-sensitive vs. case-insensitive options.