send-email: copy "config_regxp" into git-send-email.perl

send-email: copy "config_regxp" into git-send-email.perl

The config_regexp() function was added in dd84e528a3 (git-send-email:
die if sendmail.* config is set, 2020-07-23) for use in
git-send-email, and it's the only in-tree user of it.

However, the consensus is that Git.pm is a public interface, so even
though it's a recently added function we can't change it. So let's
copy over a minimal version of it to git-send-email.perl itself. In a
subsequent commit it'll be changed further for our own use.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>