builder: pxzcat: Ignore return value from posix_fadvise.

Quiets warning from Coverity.
daemon: ldm: Initialize glob_t struct before passing to glob.

Some implementations of glob might use fields in the struct, although
not glibc as far as I know.

Issue identified by Coverity.
daemon: md: Whitespace fix.
virt-diff: Free memory along error path.

Found by Coverity.
examples: Check return value from guestfs_list_filesystems.

Found by Coverity.
fish: Ignore return value from guestfs_user_cancel.

As we are in a signal handler, there's nothing else we can do.

Found by Coverity.
fish: Check return values from a couple of API calls.

Found by Coverity.