core: turn on MountAPIVFS=true when RootImage or RootDirectory are specified

core: turn on MountAPIVFS=true when RootImage or RootDirectory are specified

Lennart wanted to do this back in
https://github.com/systemd/systemd/commit/01c33c1effaa2406ff7d2a7de08a3ee87aec9fc8.
For better or worse, this wasn't done because I thought that turning on MountAPIVFS
is a compat break for RootDirectory and people might be negatively surprised by it.
Without this, search for binaries doesn't work (access_fd() requires /proc).
Let's turn it on, but still allow overriding to "no".

When RootDirectory=/, MountAPIVFS=1 doesn't work. This might be a buglet on its
own, but this patch doesn't change the situation.