Correct the accounting code for wired mappings.  The wrong field of the PVO
entry was being tested.  We were incrementing and decrementing the pmap's
wired mapping count based on whether the physical page being mapped or
unmapped was cache coherent, not whether it was a wired mapping.

Reviewed by:	nwhitehorn
Avoid relocking filedesc lock when closing fds during fdp destruction.

Don't call bzero nor fdunused from fdfree for such cases. It would do
unnecessary work and complain that the lock is not taken.

MFC after:	1 week
Don't zero fd_nfiles during fdp destruction.

Code trying to take a look has to check fd_refcnt and it is 0 by that time.

This is a follow up to r268505, without this the code would leak memory for
tables bigger than the default.

MFC after:	1 week
Make MK_GNUCXX mean "build the libstdc++ and libsupc++ libraries" and
nothing more. Force it to be "no" when MK_CXX is "no" to simplify
usage.  It no longer also means "build g++" since we no longer have a
platform where that's interesting now that pc98 no longer needs clang
and gcc, but not g++. pc98 now just uses clang after boot2 changes.