Bugfix for safeStrCpy patch for XBoard

Bugfix for safeStrCpy patch for XBoard

By a trick (temporarily redefining safeStrCpy in common.h as
((char*) (a=1/(4-3rdArgument))) ) a compile-time warning (division by 0)
could be generated for every occurrence where the size argument was equal
to 4, i.e. where the 1st argument was likely a pointer rather than an
array. These cases are now fixed. The file xoptions.c, which is the
only ne that does not #include common.h had to be fixed by making
the #define in the file itself.