Add additional userland interfaces for setting close-on-exec on fds
when creating them: mkostemp(), mkostemps(), the 'e' mode letter for
fopen(), freopen(), fdopen(), and popen().  The close-on-exec flag will
be cleared by the action created by posix_spawn_file_actions_adddup2().

Also, add support for the C11 'x' mode letter for fopen() and freopen(),
setting O_EXCL when possibly creating files.

Note: this requires kernel support for pipe2() and dup3()!

ok millert@
Add sockatmark()

ok millert@ manpage feedback jmc@