- in io_watch_have_data(), ensure that we handle incomplete lines
that don't yet have a newline by waiting for more data. That means,
if the current content of the in_buffer does not have a newline, we
wait longer.

- in io_watch_have_data(), implement (and ignore) certain commands
instead of failing the request.

- in io_watch_have_data(), no longer g_compress() the entire line.
"polkitagenthelper-pam.c" never backslash escapes the command, it
only escapes the arguments. Of course, there should be no difference
in practice, except that we don't want to handle escape sequences
in the commands.

- in io_watch_have_data(), compare SUCCESS/FAILURE literally.
"polkitagenthelper-pam.c" never appends any trailing garbage to these
commands, and we shouldn't handle that (although "polkitagentsession.c"
does).

- when io_watch_have_data() completes with success, we cannot destroy
AuthRequest right away. It probably still has data pending that we first
need to write to the polkit helper. Wait longer, and let io_watch_can_write()
complete the request.

- ensure we always answer the GDBusMethodInvocation. Otherwise, it gets
leaked.

- use NMStrBuf instead of GString.

Otherwise, we get an additional error message form the helper:

  $ nmcli --ask device wifi rescan
  System policy prevents Wi-Fi scans
  (action_id: org.freedesktop.NetworkManager.wifi.scan)
  password (user): •••••••
  polkit-agent-helper-1: pam_authenticate failed: Authentication failure
  Error: org.freedesktop.NetworkManager.wifi.scan request failed: not authorized.

The identities are in a nested dictionary of variants. We only want
to pick the "unix-user" values that we can handle.

The response is blocking, which generally is rather ugly. Let's not fix
that now, but at least rename the signal so that it clearly points this
out.

G_DECLARE_FINAL_TYPE() is glib 2.44, while we currently still only require glib 2.40.

We cannot just swallow EAGAIN and pretend that not bytes were read.

read() returning zero means end of file. The caller needs to distinguish
between end of file and EAGAIN.

NMStrBuf is not an opaque structure, so that we can allocate it on the
stack or embed it in a struct.

But most of the fields should not be touched outside of the
implementation.

Also, "len" and "allocated" fields may be accessed directly, but
they should not be modified.

Rename the fields to make that clearer.