tools/libxs: Fix CLOEXEC handling in get_dev()

tools/libxs: Fix CLOEXEC handling in get_dev()

Move the O_CLOEXEC compatibility outside of an #ifdef USE_PTHREAD block.

Introduce set_cloexec() to wrap fcntl() setting FD_CLOEXEC.  It will be reused
for other CLOEXEC fixes too.

Use set_cloexec() when O_CLOEXEC isn't available as a best-effort fallback.

Fixes: f4f2f3402b2f ("tools/libxs: Open /dev/xen/xenbus fds as O_CLOEXEC")
Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Juergen Gross <jgross@suse.com>
Acked-by: Anthony PERARD <anthony.perard@vates.tech>