This bug resulted in spurious lines with "--pretty --mode tabular",
whenever nmc_print() was called with multiple rows.

Currently, the only case where this was visible was with:

  $ nmcli --pretty general permissions

(note that "--mode tabular" is the default).

Fixes: 16299e5ac0a8882e13d42122278013b74bc6d34d

The reasons are:

- I want to locate all implmenetations of the get_fcn() handler. By
  consistently using this macro, you can just grep for the macro and
  find them all.

- all implementations should follow the same style. This macro
  enforces the same names for arguments and avoids copy&paste.

- if we are going to add or change an argument, it becomes easier.
  That's because we can easily identify all implementation and can
  change arguments in one place.

These helper function will be needed in the next commit to be earlier.
Helper functions like these, that operate solely on trival types (in
this case, converting an enum to a string), make generally sense to have
at the beginning of the source file. Because they themself have few/no
dependencies and are rather trivial and self contained.

Functions like nmc_find_connection() and nmc_find_active_connection()
can easily find multiple matching results. For example, the
"connection.id" in NetworkManager is not enforced to be unique,
so if the user adds multiple connections with the same name,
they should be all selected.

The previous API had a @pos argument, that allowed to iterate over
the results. Change that, to return all matches in a GPtrArray.

Also, extend connection-show and other places, to anticipate that
a connection might be active multiple times in any moment.

Also, let the enum2str function return a non-translated string.
The user may choose to call gettext() on it.

Beside nmc_find_connection(), which is very similar.

Also, drop unused argument @cons.

The present version of the specification is somewhat unclear at times,
Unclear points were discussed with the maintainers [1] and probably
some new version will address those.

https://www.spinics.net/lists/util-linux-ng/msg15222.html

Until then here's how the implementation copes with ambiguities
(after the discussion with util-linux maintainers):

1.) It is unclear whether multiple .schem files should override each
    other or be merged. We use the overriding behavior -- take the
    highest priority one and ignore the rest.

2.) We assume "name.schem" is more specific than "@term.schem".

3.) We assume the "Color name" are to be used as aliases for the color
    sequences and translate them to ANSI escape sequences.

4.) The "Escape sequences" are of no use since the specification
    pretty much assumes an ANSI terminal and none of the sequences make
    any sense in ANSI color codes. We don't support them.
    accept that.

5.) We don't implement TERMINAL_COLORS_DEBUG because it's unspecified
    what should it do.