NMParamSpecSpecialized existed basically to provide a version of
GParamSpecBoxed that could compare dbus-glib-valued properties
correctly.

However, g_param_values_cmp() was only used by NM directly in one
place (NMSetting's compare_property()), and we don't actually need to
indirect through GParamSpec there; we could just call
NMParamSpecSpecialized's value-comparison function directly.

So, change all NMParamSpecSpecialized properties to GParamSpecBoxed,
rename the _gvalues_compare() function it used to
"nm_property_compare()", and use that from NMSetting.

(g_param_values_cmp() also gets used internally by
g_param_value_defaults(), but all NMParamSpecSpecialized properties
have a default value of NULL, so GParamSpecBoxed's pointer-equality
check will do the job just fine there.)