Fix a problem where 'make installworld' will fail and leave the system in a messy state *if* the user is upgrading from a system which has no /libexec to a system which builds a DYNAMICROOT, and if that user has set DISTDIR (as documented for ports, but it turns out that the same variable name is used for a completely unrelated purpose in 'make release').
Fix a problem where 'make installworld' will fail and leave the
system in a messy state *if* the user is upgrading from a system
which has no /libexec to a system which builds a DYNAMICROOT, and
if that user has set DISTDIR (as documented for ports, but it turns
out that the same variable name is used for a completely unrelated
purpose in 'make release').

There are other possible fixes for this issue, and ru@ may later
decide to commit one of those fixes.  I just wanted some fix in
ASAP, and this is the fix that I have tested.

Reviewed by:	bde, imp, and ru

jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase