netcmd: Improve domain backup targetdir checks

netcmd: Improve domain backup targetdir checks

+ Added check that specified targetdir is actually a directory (if it
exists)
+ Deleted a redundant 'Creating targetdir' check that would never be hit
+ Move code into a separate function so we can reuse it for offline
backups (which take a different set of parameters, but still have a
targetdir)

Signed-off-by: Tim Beale <timbeale@catalyst.net.nz>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>
Reviewed-by: Gary Lockyer <gary@catalyst.net.nz>