workaround for error message for missing pieces used in variants and initstring bugfix

workaround for error message for missing pieces used in variants and initstring bugfix

error messages showed for non-existing pieces showed up for old user supplied pixmaps. Loading a pixmap of the king for the missing pieces now.

Another bad bug surfaced: the -initString argument does not work in any
XBoard version! (Including 4.2.7b) The escape sequences for linefeed in
the string are not understood: -initString "new\nrandom\n" actually sends
the \ and n to the engine (which then does not recognize the line as a
valid command)!

So I also added a patch to expand escape sequences in the InitString and
ComputerString command-line options.