fstab-util: introduce fstab_enabled() helper function

fstab-util: introduce fstab_enabled() helper function

And refuse to parse fstab when 'fstab=no' is specified in the kernel
command line.

When 'fstab=no' is specified in the kernel command line, fstab-generator
does not parse fstab and will not create e.g. /boot or /efi mount entry
even if fstab contains entries for the mount points. However, gpt-auto
generator may parse fstab file, and adjust or ignore mounts for EFI or
XBOOTLDR partitions based on the fstab file.

This makes gpt-auto also ignore fstab entries if 'fstab=no' is set in
the kernel command line.