inspection: Cleanup iteration over fstab entries in inspect_fs_unix.c

inspection: Cleanup iteration over fstab entries in inspect_fs_unix.c

Select non-comment labels using an augeas path to return the correct
nodes in the first instance, rather than applying a regular expression
to all results.

There is no functional change to the code.

RWMJ:
 - Rebased to current HEAD.
 - Move variable decls to top of function.
 - Some whitespace changes.
 - Remove check for errors from snprintf since it can never happen.