mailsplit: make PATH_MAX buffers dynamic

mailsplit: make PATH_MAX buffers dynamic

There are several PATH_MAX-sized buffers in mailsplit, along
with some questionable uses of sprintf.  These are not
really of security interest, as local mailsplit pathnames
are not typically under control of an attacker, and you
could generally only overflow a few numbers at the end of a
path that approaches PATH_MAX (a longer path would choke
mailsplit long before). But it does not hurt to be careful,
and as a bonus we lift some limits for systems with
too-small PATH_MAX varibles.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>