libsigsegv: rename patches to the new convention and convert to Git format

This commit renames the libsigsegv patches to the new convention, and
converts them to Git formatted patches. Along the way, it also aligns
the description of the different patches, in a preparation for
upstream submission.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>