Signed-off-by: Thomas Haller <thaller@redhat.com>

The change to per-domain log levels means that when setting just the
level, we need to re-set the log level for each domain (since it's the
"logging" bit array that actually determines what gets logged).
nm_logging_setup() was dealing correctly with domains=NULL, but not
domains="" (which is what happens when it is invoked with only a level
via D-Bus), so doing "nmcli gen log level DEBUG" would change the
"default" log level, but leave all of the domains still at their
previous level:

danw@laptop:NetworkManager> nmcli g log
LEVEL  DOMAINS
INFO   PLATFORM,RFKILL,ETHER,WIFI,BT,MB,DHCP4,DHCP6,PPP,IP4,IP6...

danw@laptop:NetworkManager> nmcli g log level DEBUG
danw@laptop:NetworkManager> nmcli g log
LEVEL  DOMAINS
DEBUG  PLATFORM:INFO,RFKILL:INFO,ETHER:INFO,WIFI:INFO,BT:INFO...

nm_logging_level_to_string() returned a const string, but
nm_logging_domains_to_string() returned a malloced one, which was
getting leaked in impl_manager_get_logging(). Fix this by making them
both malloced, and freeing as needed.

If the command line or NetworkManager.conf mentions a non-existent
domain, just print a warning and ignore it. That way if you switch to
using an older NM that doesn't have that domain, it will still work.

Allow specifying log domains like "DEFAULT,WIFI:DEBUG" to override the
log level on a per-domain basis.

Rather than having separate nm_logging_level_enabled() and
nm_logging_domain_enabled(), have just nm_logging_enabled() that
checks both.