mount: default startup dependencies and default network ones are orthogonal

mount: default startup dependencies and default network ones are orthogonal

Regardless of whether a mount is setup in initrd or int the main system,
the network default dependencies _netdev should still be honored.

IOW if a mount unit use the following options "x-initrd.mount,_netdev", it
should be ordered against initrd-fs.target, network.target,
network-online.target.

/dev/vdb1 /mnt ext4 x-initrd.mount,_netdev defaults 0 0

Before this patch:

Before=umount.target initrd-fs.target
After=system.slice sysroot.mount dev-vdb1.device -.mount systemd-journald.socket blockdev@dev-vdb1.target

After this patch:

Before=initrd-fs.target umount.target
After=network-online.target -.mount blockdev@dev-vdb1.target dev-vdb1.device sysroot.mount system.slice network.target systemd-journald.socket

(cherry picked from commit 2ec15c4f8a288d4f2e92ba2b8586736b2a07b9ea)