libmd: Install libmd.so.6 into /lib again after SVN r338270

[jilles]

Summary

Including <src.opts.mk> sets SHLIBDIR, so the below SHLIBDIR?= has no
effect.

The other lib/ Makefiles solve this problem by moving the SHLIBDIR
assignment to before .include <src.opts.mk>.

Test Plan

make buildworld with SVN r338310, make installworld (notice that
/lib/libmd.so.6 has an old timestamp and /usr/lib/libmd.so.6 exists),
apply patch, make buildworld -DNOCLEAN, make installworld (notice that
/lib/libmd.so.6 has a recent timestamp and /usr/lib/libmd.so.6 is gone),

[allanjude]

Fixes zfs @ mountroot