mask off SOCK_CLOEXEC in socketpair() emulation

In the unlikely situation that we have SOCK_CLOEXEC but are emulating
socketpair(), having the bit set could confuse the emulation.  It's not
worth actually implementing SOCK_CLOEXEC logic for this situation, so
just mask it off.  The higher-level logic of PerlSock_socketpair_cloexec()
will handle the bit being ineffective.