This is a huge refactoring in attempt to 1.) reduce the horrible redundancy in
the connection addition path and 2.) reduce confusion between various sources
of property value (command line, properties, interactive mode).

* The conversions from the strings was done all over the place:
  settings.c already does for all sensible properties.
  The rest is removed.

* The validations are done randomly and redundantly:
  server does some validation, and per-property client validations
  useful for interactive mode are done in settings.c
  The rest is removed.

* The information about defaults and required options was redundantly
  scattered in per-type completion functions and interactive mode
  questionnaries. This is now driven by the option_info[] table.

In general, we do our best to just map the command line options to
properties and allow mixing them. For the rest there's the
check_and_set() callbacks (basically to keep compatibility with previous
nmcli versions). This this is now all possible:

$ nmcli c add type ethernet ifname '*'
  This always worked

$ nmcli c add type bond-slave save no -- connection.autoconnect no
  The "save" and "--" still work

$ nmcli c add connection.type ethernet ifname eth0
  Properties can now be used

$ nmcli c add type ethernet ip4 1.2.3.4 mac 80:86:66:77:88:99 con-name whatever
  There's no implementation mandated order of the properties (the type
  still must be known to determine which properties make sense)

$ nmcli --ask c add type ethernet ip4 1.2.3.4 mac 80:86:66:77:88:99 con-name whatever
  The interactive mode asks only for properties that weren't specified
  on command line