fstab-generator: add x-systemd.mount-timeout (#4603)

This adds a new systemd fstab option x-systemd.mount-timeout. The option
adds a timeout value that specifies how long systemd waits for the mount
command to finish. It allows to mount huge btrfs volumes without issues.

This is equivalent to adding option TimeoutSec= to [Mount] section in a
mount unit file.

fixes #4055