Fix Savannah bug #19656: rationalize our use of case-insensitive string

Fix Savannah bug #19656: rationalize our use of case-insensitive string
comparison functions to always use POSIX strcasecmp().  For non-POSIX
systems that use other functions (strcmpi or stricmp) use a macro to alias
strcasecmp to those.  If we can't find any of them (VMS, plus whatever
UNIX doesn't have them) then define our own version in misc.c.