x11/libxshmfence: don't leak /tmp/shmfd* files with O_CLOEXEC

• Both mkostemp() and mkstemp() need explicit call to unlink()
• Unobfuscate fallback if both O_TMPFILE and mksotemp() are N/A
• O_TMPFILE (Linux-only) appeared after O_CLOEXEC, no need to check

PR: 217676 (for tracking)
MFH: 2017Q3 2017Q2