fstab-generator: handle systemd.swap= command-line argument

Don't generate swap units if set to false

The inverse of this argument is present as "noswap" on Debian sysvinit

Ref:
https://salsa.debian.org/debian/sysvinit/blob/4422988cb41c3022eee7444378cac3b2e36eac28/debian/vars.sh#L34
https://salsa.debian.org/debian/sysvinit/blob/4422988cb41c3022eee7444378cac3b2e36eac28/debian/src/initscripts/etc/init.d/mountall.sh#L78

Fixes https://github.com/systemd/systemd/issues/6686