Mark more file descriptors close-on-exec

Mark more file descriptors close-on-exec

I noticed a couple of spots in gdb that were opening files but not
marking the file descriptors as close-on-exec.  This patch fixes
these.

There are still a few more of these, but they are in code that I can't
compile, so I'd prefer not to touch.

gdb/ChangeLog
2018-09-23  Tom Tromey  <tom@tromey.com>

	* ctf.c (ctf_start): Use gdb_fopen_cloexec.
	* common/scoped_mmap.c (mmap_file): Use gdb_open_cloexec.