Fix a race between kern_proc_{o,}filedesc_out and fdescfree leading
to use-after-free.

fdescfree proceeds to free file pointers once fd_refcnt reaches 0, but
kern_proc_{o,}filedesc_out only checked for hold count.

MFC after:	3 days
Move the part in bsd.own.mk that sets -Wno-c++11-extensions for clang to
bsd.sys.mk, where it really belongs.  This also causes the flag to get
added when clang is *not* the default system compiler, but is still
used, e.g. by setting WITH_CLANG_IS_CC manually.

MFC after:	3 days