fish: Don't fail if some mountpoints in /etc/fstab are bogus (RHBZ#668574).

fish: Don't fail if some mountpoints in /etc/fstab are bogus (RHBZ#668574).

Fix guestfish (and other C tools) so that they ignore errors
when /etc/fstab contains bogus entries.

Update the documentation for inspect-get-mountpoints to emphasize
that callers must be aware of this when mounting the returned
values.

Add a regression test.

Update the example code ("inspect_vm") to reflect the way this
API ought to be called.

For more detail see:
https://bugzilla.redhat.com/show_bug.cgi?id=668574
(cherry picked from commit 0c60e4d9dd6549c2135699490ba8a9ec1dd50ab9)