devel/valgrind-devel: update to Paul Floyd's FreeBSD valgrind branch

This branch rebases us forward to Valgrind 3.17.0, incredibly current as
of today. It also fixes a vast array of known issues, making valgrind-devel
actually functional on 13.0-CURRENT and fixing most of the regression tests
on clang/amd64.

All patches previously in the port have been incorporated into this branch.

There are still some open issues remaining:

• https://github.com/paulfloyd/freebsd_valgrind/issues

Paul has summarized the regtest status here:

• https://github.com/paulfloyd/freebsd_valgrind/wiki/Regtest-status

This port is believed to be largely functional enough to get wider testing.
Many, many thanks to the excellent work of Paul Floyd to modernize our
valgrind port.

Pass MAINTAINER over to Paul, as he intends to continue working on an effort
to upstream valgrind/FreeBSD support.

Approved by: zeising (maintainer)
Differential Revision: https://reviews.freebsd.org/D25452