Only set -Wno-c++11-extensions if we're building with clang, so bootstrapping
clang with gcc doesn't fail.
Don't delete c++filt when doing a make delete-old if GCC is not built but
C++ is.
Add WITH_GCC alongside WITHOUT_GCC.
Remove the comment from WITHOUT_GCC about this not working without an
alternate toolchain, we have clang now.
Regenerate after unhooking gcc/g++ from the default build for some arches.
libc: Use SOCK_CLOEXEC for various internal file descriptors.

This change avoids undesirably passing some internal file descriptors to a
process created (fork+exec) by another thread.

Kernel support for SOCK_CLOEXEC was added in r248534, March 19, 2013.
Retire netisr.netisr_direct and netisr.netisr_direct_force sysctls.
These were used to control/export dispatch policy but they're not anymore.
This commit cannot be MFC'ed to 9 because old netstat(9) binary relies
on such sysctl to work. On the other hand, there's no real reason to
keep'em around in 10.
Bump .Dd after r255307 and r255310

Requested by:	joel