XBoard: make messages compatible with xboard protocol.

Introduces the "xboard" global var, but only yhe last commit will set
it, when support is complete.

Some non-compliant utterances had to be disarmed by commenting them
out with a leading '#'.

YD: split original patch into a series
XBoard: deal with semantic differences in "post" between xboard and xshogi protocols

YD: split original patch into a series
XBoard: handle xboard-specific commands

YD: split original patch into a series
YD: parenthesized to avoid warning
XBoard: declare feature sigint=0 to avoid getting killed by the GUI.
XBoard: select an xboard-compatible implementation of "level" command

YD: split original patch into a series
YD: rebased onto recent refactorings
XBoard: use xboard v2-compatible move output ("move" command, no milliseconds)

YD: split original patch into a series
YD: simplified slightly
XBoard: teach about xboard move notation.

Slight unfortunate side-effect: pat2inc requires a declaration of the xboard
flag, although it should really be constant there.
XBoard: switch "black" and "white" in commands and names, activate "xboard" command.

YD: split original patch into a series
Advertise XBoard as official replacement for XShogi.