1. When downloading metadata files, make sure we only download each
file once; without this fix, "freebsd-update fetch" will fail the first
time it is run if there have been no updates yet for the installed
release.
2. If the FOO kernel is installed in /boot/kernel instead of /boot/FOO
and the /boot/FOO directory does not exist, don't try to update
/boot/FOO. This is an issue only where an update involves adding a new
kernel module.
3. When removing files and directories, operate in reverse
lexographical order, in order to ensure that files are removed before
the directory which contains them.
MFC after: 3 days
