fstab-generator: avoid mangling of non-path mount source and dest

fstab-generator: avoid mangling of non-path mount source and dest

This can invalidate otherwise valid source paths with trailing slashes,
such as "host:/" in the case of a network mount.

Based on a patch from Dave Reisner <dreisner@archlinux.org>, which
removed the slash mangling entirely.

Changed it to match on the leading slash to exclude non-path values.