New API: remount: Allow rw flag to be adjusted on mounted filesystem.
events: Avoid event handler limit if program allocates and deallocates handlers.
inspection: Use guestfs_is_{dir,file} in preference to guestfs_exists.

Where possible, using this is more accurate, and it helps when
trying to do fuzz testing.
perl: Add rules + environment variables so Perl code can be valgrinded.
inspection: Add a fuzz-test of inspection.