core: add a per-unit setting MountAPIVFS= for mounting /dev, /proc, /sys in conjuncti...

core: add a per-unit setting MountAPIVFS= for mounting /dev, /proc, /sys in conjunction with RootDirectory=

This adds a boolean unit file setting MountAPIVFS=. If set, the three
main API VFS mounts will be mounted for the service. This only has an
effect on RootDirectory=, which it makes a ton times more useful.

(This is basically the /dev + /proc + /sys mounting code posted in the
original #4727, but rebased on current git, and with the automatic logic
replaced by explicit logic controlled by a unit file setting)