vmimage.subr: Add ability to install src in VM image

[kgalazka]

Summary

In some cases having a src tree in a VM image is convenient
for development or debugging. Add a WITH_SRC variable,
which, when set, will cause the vm-release target to include
FreeBSD-set-src in the list of packages installed in an image.

Signed-off-by: Krzysztof Galazka <[e-mail]>

Sponsored by: Intel Corporation

[kgalazka]

It's submitted also to Forgejo: https://ron-dev.freebsd.org/FreeBSD/src/pulls/32, but I guess Phabricator is still preferred.

[kgalazka]

@cperciva Could you please take a look?

[cperciva]

LGTM.