smb: Improve enumeration performance

The samba backend calls stat for each file during the enumeration job.
Unfortunately, this is significantly slower with the recent samba versions.
However, smbc_readdirplus2() has been recently added and it can be used
to improve the performance as all the necessary info is transferred in one
network request. Let's use smbc_readdirplus2() if available.

See: https://gitlab.com/samba-team/samba/merge_requests/302

Relates: #306
3 jobs for master in 2 minutes and 20 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Test
passed #490222
fedora

00:02:20

manual #490223
allowed to fail manual
gnome-build-meta-manual
manual #490224
allowed to fail manual
update-image