CECP: set feature 'debug'

[unserializable]

Lo and behold, apparently the debug comments output prefixed by '#' should actually be only sent after setting feature `ďebug=1`` -- https://www.gnu.org/software/xboard/engine-intf.html#9

debug (boolean, default 0)
    If debug=1, it means the engine wants to send debug output prefixed by '#', 
which WinBoard should ignore, except for including it in the winboard.debug file. 
As this feature is added to protocol 2 ony late, so that not all protocol-2 supporting
versions of WinBoard might implement it, it is important that engines check if WinBoard
accepts the feature. If the feature is rejected, engines must refrain from sending the
debug output, or do so at their own risk. 

Considering that so far the # debug comments are used whenever running under CECP is recognized, this has been working very well -- bring this in line with the specifications.

From http://hgm.nubati.net/newspecs.html

When an engine sends stuff the GUI does not understand, the result is in principle undefined, but GUI should
make its best attempt to ignore the entire non-compliant line. Valid reasons for not ignoring the line are
that the protocol has been extended to now attach a meaning to the engine utterance, or to treat it as a
different valid command, in order to cater to known engine bugs (like recognizing commands with typos). The
latter is really bad (but alas common) practice: XBoard even takes any line containing the word "Draw" as an
"offer draw" command. When an engine wants to print non-compliant messages (e.g. to aid debugging), it should
better use the debug feature to 'disarm' these message by starting them with '#' to make sure they cannot have
unexpected results. And even this might not protect you when the line contains system error messages like "not
found" or "permission denied"! 

This debug feature originated in 2008, post from July "WB protocol for engine comments" has HGM mentioning this as official mechanism from "WinBoard 4.3.13 on".

[unserializable]

After adding the debug=1, it can be seen that accepted debug is also sent by Xboard 4.9.1.

$ xboard -debug -debugfile 113-2.debug -fcp ./monchester
$ grep -E "[<>]first" 113-2.debug 
313 >first : xboard
522 <first : # Monchester 1.0.1-24-g97c50f4 ~(4403 kN/s)
522 <first : command : # received xboard
522 <first : feature myname="Monchester 1.0.1-24-g97c50f4" name=1 setboard=1 ping=1 debug=1 edit=0 memory=0 usermove=0 analyze=0 colors=0 sigint=0 sigterm=0 done=1
522 >first : accepted myname
522 >first : accepted name
522 >first : accepted setboard
522 >first : accepted ping
522 >first : accepted debug
522 >first : rejected edit
522 >first : accepted memory
522 >first : accepted usermove
522 >first : accepted analyze
522 >first : accepted colors
522 >first : accepted sigint
522 >first : accepted sigterm
522 >first : accepted done
523 >first : new
524 >first : level 0 1 1
524 >first : post
524 >first : hard
524 >first : ping 1
524 <first : #8:  R N B Q K B N R 
524 <first : #7:  P P P P P P P P 
524 <first : #6:  - - - - - - - - 
524 <first : #5:  - - - - - - - - 
524 <first : #4:  - - - - - - - - 
524 <first : #3:  - - - - - - - - 
524 <first : #2:  p p p p p p p p 
524 <first : #1:  r n b q k b n r 
524 <first : #-------------------
524 <first : #    A B C D E F G H       rnbqkbnr/pppppppp/8/8/8/8/PPPPPPPP/RNBQKBNR w KQkq - 0 1
524 <first : pong 1

[unserializable]

Commit considered enough, fine-grained parsing of accepted debug not considered necessary.