uat: Minor updates
Add PT_TXTMOD_DISSECTOR to the casees handled by uat_fld_tostr. Reorder the cases to more closely match putfld, which uat_save uses.
Get rid of an unnecessary strdup of a GString.
uat_load_str dupes the string entry passed in, so that can be const.
Note that uat_fld_tostr doesn't do what the documentation says, though sharkd_session uses it as-is and puts the results in JSON instead of the UAT CSV format.