add a new tool for creating transient mount and automount units

add a new tool for creating transient mount and automount units

This adds "systemd-mount" which is for transient mount and automount units what
"systemd-run" is for transient service, scope and timer units.

The tool allows establishing mounts and automounts during runtime. It is very
similar to the usual /bin/mount commands, but can pull in additional
dependenices on access (for example, it pulls in fsck automatically), an take
benefit of the automount logic.

This tool is particularly useful for mount removable file systems (such as USB
sticks), as the automount logic (together with automatic unmount-on-idle), as
well as automatic fsck on first access ensure that the removable file system
has a high chance to remain in a fully clean state even when it is unplugged
abruptly, and returns to a clean state on the next re-plug.

This is a follow-up for #2471, as it adds a simple client-side for the
transient automount logic added in that PR.

In later work it might make sense to invoke this tool automatically from udev
rules in order to implement a simpler and safer version of removable media
management á la udisks.