Implement less kludgy way for conditional string options

Implement less kludgy way for conditional string options

A new option -replace can now define a string, and when the existing value
of the next encountered string option does not exactly match that, the
latter is ignored. The @@@@@ kludge for conditionally replacing empty
strings is removed, as this can now be achieved through -replace "".
The value of -replace is reset to NULL (not ""!) as soon as it has been
used for comparison, irrespective of the result. This makes -replace a
'hyper-volatile' option.
 The conditional setting of UCI2WB as -uxiAdapter in xboard.conf is now
changed to use this mechanism.