nmcli/connections: fix setting ifname with "--ask c add"

We almost always do the wrong thing in interactive add:

The software devices generally require an interactive name, but we don't
insist of asking for them; treating them as optional:

  $ nmcli -a c add type dummy
  There is 1 optional setting for General settings.
  Do you want to provide it? (yes/no) [yes]

For some interface types (bridges, bonds, ...) we make up a name, presumably
for historical reasons. But we don't give the user an option to modify
them:

  $ nmcli -a c add type bridge
  <not asking for interface name at all>
  There are 9 optional settings for Bridge device.
  Do you want to provide them? (yes/no) [yes]

This fixes the above use cases -- still set the default, but be sure to
ask:

  $ nmcli -a c add type dummy
  Interface name:

  $ nmcli -a c add type bridge
  Interface name [nm-bridge1]:

Beautiful.