Refactor the import code completely.

- openvpn supports quoting and escaping. The parsing code sometimes
  considered quotes (e.g. see unquote() function and the line
  strcmp (file, "'auto'")).
  However, we must consider escaping/quoting while splitting the
  line into invidivual args. get_args() did that wrong in that
  it right away called g_strsplit_set().
  Now, split the line right away with args_parse_line().

- don't print any g_warning() during import but instead return
  a proper error. Import will be called from nmcli and it is bad
  that a library spams stdout/strerr.
  This makes import code much less forgiving. Now instead of printing
  a warning, import fails.

- also, be more strict about the number of arguments that an option
  has. Now, if the number of arguments mismatches, fail with error.

- optimally, we would like to fail for every option that we don't
  understand. It is questionable to signal success during import
  when certain parts are not understood. Anyway, that did not change
  yet, unrecognized options are still silently ignored. But we got
  stricter with arguments that we understand and where we now enforce
  the right number of arguments.

- adjust the blob-reading code to be similar to what openvpn does.
  E.g. the end-tag is detected with allowing leading whitespace and
  and trailing message.

- also, don't treat contents as a NUL terminated C string, but consider
  the length. Lines are split at either '\0' or '\n'. E.g. a inline blob
  element with '\0' will still be written out as read-in.