MSG_CMSG_CLOEXEC has not been working since 2015 (SVN r284380) because
_finstall expects O_CLOEXEC and not UF_EXCLOSE as the flags argument.
This was probably not noticed because we don't have a test for this flag
so this commit adds one. I found this problem because one of the
libwayland tests was failing.
Fixes: ea31808c3b07 ("fd: move out actual fp installation to _finstall")