opal/util/fd.c: add new convenience function for setting FD_CLOEXEC

Paul Hargrove pointed out that Stevens tells us that we should
FD_GETFL before FD_SETFL.  And so we shall.

Make a new convenience function to do this (opal_fd_set_cloexec()),
just so that we don't have to litter this 2-step process throughout
the code.

Refs #4550