Do some minimal verification of hostnames that come in via D-Bus, for
length and content.  Otherwise we'd get as far as asking glibc to set
the system hostname, which would reject us.

Pretty much the same thing as PERMISSION_DENIED.

When NM was registering all of its enum types by hand, it was using
NamesLikeThis rather than the default names-like-this for the "nick"
values. When we switched to using glib-mkenums, this resulted in
dbus-glib using different strings for the D-Bus error names, causing
compatibility problems.

Fix this by using glib-mkenums annotations to manually fix all the
enum values back to what they were before. (This can't be done in a
more automated way, because the old names aren't 100% consistent. Eg,
"UNKNOWN" frequently becomes "UnknownError" rather than just
"Unknown".)

Rather than generating enum classes by hand (and complaining in each
file that "this should really be standard"), use glib-mkenums.

Unfortunately, we need a very new version of glib-mkenums in order to
deal with NM's naming conventions and to fix a few other bugs, so just
import that into the source tree temporarily.

Also, to simplify the use of glib-mkenums, import Makefile.glib from
https://bugzilla.gnome.org/654395.

To avoid having to run glib-mkenums for every subdirectory of src/,
add a new "generated" directory, and put the generated enums files
there.

Finally, use Makefile.glib for marshallers too, and generate separate
ones for libnm-glib and NetworkManager.