Unbreak installation of the rtld tests.

X-MFC-With:	r267679
Make sure that the sub-makes for unwind.h start from the CURDIR
(/usr/src) tree rather than the OBJDIR (/usr/obj) tree. This fixes
broken incremental builds with the canonical MAKESYSPATH workaround
of .../share/mk. This is a gross kludge.
Now that vm_map_insert() sets MAP_ENTRY_GROWS_{DOWN,UP} on the stack entries
that it creates (r267645), we can place the check that blocks map entry
coalescing on stack entries in vm_map_simplify_entry() where it properly
belongs.

Reviewed by:	kib
Continue the crusade towards a dev_clone()-free kernel, removing its
usage from dtrace. The dtrace code already uses cdevpriv(9) since FreeBSD
8, so this change should be quite harmless.

Reviewed by:	markj
Approved by:	markj
MFC after:	never
Merge remote-tracking branch 'upstream/master'