On the windows VFS, do not try to make InterlockedCompareExchange an
overloadable function, since sometimes it is a macro.
Fix an obsolete comment in the func.c source file.  No changes to code.
Fix harmless compiler warnings.  In the command-line shell, report if the
".system" command returns a non-zero result.
Fix two more harmless compiler warnings.  Make sure the fts3_unicode2.c file
is in sync with mkunicode.tcl.
A couple more harmless compiler warnings eliminated.