sysext: Add --mutable mode flag

sysext: Add --mutable mode flag

The flag takes "auto" or "import" or a boolean value.

"auto" causes systemd-sysext to make a decision about mutability of the merged
hierarchy based on existence of the upper directory in
`/var/lib/extensions.mutable/${hierarchy}`.

"import" causes the existing upper dir to be actually used as another lower
dir, which results in read-only merged hierarchy.

True value makes systemd-sysext to create the upper dir if it's missing and to
make the merged hierarchy mutable.

False value makes systemd-sysext to ignore upper dir completely, and create a
read-only merged hierarchy.

The default is false value.