FAST_DEPEND: Use .dinclude to enable full .depend logic in bmake.

The inclusion of .MAKE.DEPENDFILE (.depend) has special logic in make
to ignore stale/missing dependencies. bmake 20160220 added a '.dinclude'
directive that uses the special logic for .depend when including the file.

This fixes a build error when a file is moved or deleted that exists in a
.depend.OBJ file. This happened in r292782 when sha512c.c "moved" and an
incremental build of lib/libmd would fail with:

make: don't know how to make /usr/src/lib/libcrypt/../libmd/sha512c.c. Stop

Now this will just be seen as a stale dependency and cause a rebuild:

make: /usr/obj/usr/src/lib/libmd/.depend.sha512c.o, 13: ignoring stale .depend for /usr/src/lib/libcrypt/../libmd/sha512c.c
--- sha512c.o ---
...

This rebuild will only be done once since the .depend.sha512c.o will
be updated on the build with the -MF flags.

This also removes -MP being passed for the .depend.OBJ generation (which
would create fake targets for system headers) since the logic is no
longer needed to protect from missing files.

Sponsored by: EMC / Isilon Storage Division