Update share/mk files from bmake

Update to the latest makefiles etc from bmake.
Mostly this just replaces sjg license with an SPDX tag.

There are also some improvements to meta2deps* and optimizations
to leverage POSIX shell features in some target scripts.
Default isPOSIX_SHELL to ':' in sys.mk to enable these.

Use :sh1 in M_type if possible.

bsd.progs.mk has diverged too much to touch beyond making the
SPDX tag update.

Reviewed by: stevek
Differential Revision: https://reviews.freebsd.org/D54150