r338270 had the side effect of no longer installing libmd.so into /lib.
For users who have a seperate zfs mount of /usr or /usr/lib, this will
cause dynamic loading failures when attempting to execute zfs mount on
bootup. E.g. the system won't boot.

Including <src.opts.mk> sets SHLIBDIR, so SHLIBDIR?= has no
effect. The other lib/ Makefiles solve this problem by moving the
SHLIBDIR assignment to before .include <src.opts.mk>.

Submitted by: jilles
Reviewed by: allanjude
Approved by: re (rgrimes)
Differential Revision: https://reviews.freebsd.org/D16910