src: use closefrom() for mass closing of FDs

src: use closefrom() for mass closing of FDs

On FreeBSD 12 the default ulimit settings allow for 100,000
open file descriptors. As a result spawning processes in
libvirt is abominably slow. Fortunately FreeBSD has long
since provided a good solution in the form of closefrom(),
which closes all FDs equal to or larger than the specified
parameter.

Reviewed-by: Ján Tomko <jtomko@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>