Log Message:
pkg_install-20240125: Extend `-m' syntax to allow OPSYS too.

- If there's no slash `/', take it all as ${MACHINE_ARCH}.

- If there is a slash, then split it by `/' and ` ' into:

        ${OPSYS}/${MACHINE_ARCH} ${OPSYS_VERSION}

  For example:

        NetBSD/aarch64 10.0

All the variables are restricted to lie in a safe set [a-zA-Z0-9._-],
so the notation can be extended later.

No change to existing syntax (no MACHINE_ARCH has `/' in it, or
anything outside the safe set), and `-m' is generally only used with
cross builds anyway, so this shouldn't break existing cross builds
and should have no impact on native builds.

Proposed on tech-pkg:
https://mail-index.netbsd.org/tech-pkg/2024/01/13/msg028825.html