META_MODE: Allow MAKEOBJDIRPREFIX to work more closely to its traditional behavior.

The preferred way to modify the object directory root is to use OBJROOT.
However, setting OBJROOT to ${MAKEOBJDIRPREFIX}/${SRCTOP}/ effectively behaves
as expected.

The problem with this before was that setting OBJROOT to contain SRCTOP
resulted in a recursive replacement (/usr/obj/usr/obj/usr/src/). Anchoring to
the start of the path for replacing SRCCTOP in CURDIR resolves this by
avoiding replacing SRCTOP when CURDIR is within the OBJDIR.

Sponsored by: EMC / Isilon Storage Division