And make it incredibly slow at the same time.

It allows us to clean up the nmcli "c add" section considerably.

We list the old-fashioned aliases in a separate section that applies to both
"nmcli c add" and "nmcli c modify".

The section is now nicely cross-linked with nm-settings in HTML
rendering.

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

Complete the property as we parse the list of properties. This makes it
possible to actually complete an unfinished property.  E.g:

  $ nmcli --complete c modify enp0s25 +ipv6.addr
  +ipv6.addresses +ipv6.addr-gen-mode

Make property autocompletion take a prefix and modifier flags.

This will make it easier to complete an unfinished property name
(possibly accompanied by a modifier) without shell trickery.

Will be useful to pass around the complete flag.

nmcli bash autocompletion leveraged on "nmcli connection edit", "print"
to retrieve the specific properties of a connection. Anyway, the
interactive editor is smart and just prints the used components, so in a
connection where 802.1x is not enabled we had no autocompletion.
Solved adding an "hidden" command "nmcli --complete connection modify"
as suggested in bgo #724860 in order to retrieve ALL the available
properties for use in autocompletion.
Here patch from L.Rintel has been merged to make che --complete option
global to nmcli (first version was local to "connection modify").

https://bugzilla.gnome.org/show_bug.cgi?id=724860
https://bugzilla.redhat.com/show_bug.cgi?id=1301226

* no need to check HELP_ONLY_AS_FIRST var as when --help option is passed
  _nmcli_compl_OPTIONS will return 0, falling in the general case that
  will trigger end of autocompletion

* clanup local var declaration in _nmcli func:
  - remove dupliated OPTIONS_MANDATORY declaration
  - init HELP_ONLY_AS_FIRST on declaration
  - order vars for common prefix