Various cleanups:
- after detecting the modifier, remove it from the string right away.
It's redundant and confusing to do it later.
- rename variables and move to inner scope.
- don't use g_str_split() to split the property name at the
first dot. strchr() is sufficient.
Also, now that we strip the modifier from option early, they start also
working for aliases. There is no need to not support (or behave
differently) w.r.t. whether aliases support modifiers or not.
This fixes:
$ nmcli connection modify r +ip4 192.168.5.2/24
Error: invalid <setting>.<property> 'ip4'.