bmake: rethink default mk search path again
Talking with swildner, we don't want the magic token to come before
/usr/share/mk because that would case the host make (after bootstrapping) to
seek files /usr/src/share/mk rather than the world mk files.
Also, it appears that the magic token won't even help with pkgsrc support
the mk files are in another branch (e.g. /usr/pkg/share/mk when the token
would look in /usr/pkgsrc/share/mk). We really don't have a use for
.../share/mk, but putting it after /usr/share/mk won't hurt anything.
Leave .../share/mk in place just in case it becomes a useful capability later.