Added libxrdp_get_channel_count()

[matt335672]

Fixes #1775

The particular error focussed on here this :-

[yyyymmdd-hh:mm:ss] [ERROR] libxrdp_query_channel - Channel out of range 4

Previous to this PR, the only way that xrdp code had to iterate over the channels was to call libxrdp_query_channel() with channel ID values increasing from zero until the call failed. This method would always result in the above message being generated when the maximum channel ID was exceeded. Although the message is harmless it is misleading in the sense that no error has actually occured.

The PR adds the call libxrdp_get_channel_count() so that code which needs to iterate over the channels can use a for loop to do this. This occurs in two places:-

1. xrdp_mm_trans_send_channel_setup() iterates over the channels when sending them to chansrv
2. xrdp_wm_init() iterates over the channels when processing the [Channels] section in xrdp.ini

In both cases, the resulting code now reads more easily.

Two other cleanups have been incorporated:-

1. The channel ID parameter to libxrdp_query_channel() has been renamed from index to channel_id for consistency with other channel calls.
2. A couple of g_writeln() calls in xrdp_wm.c have been replaced with a single call to the LOG() macro.

This PR conflicts a little with #1742 - that PR also addresses these particular errors:-

[yyyymmdd-hh:mm:ss] [ERROR] libxrdp_query_channel - Channel 0 name rdpdr

[aquesnel]

Looks good

[metalefty]

LGTM, too.