Fix StrCaseCmp() to avoid calling smb_panic() on invalid multibyte strings.

Fix StrCaseCmp() to avoid calling smb_panic() on invalid multibyte strings.

This fix results in
 - we no longer use fixed-size buffers in StrCaseCmp (previously limited to
   a pstring)
 - we return strcmp(s, t) if either of the strings is invalid
 - for non-ascii cases, we call iconv twice, not 4 times.

The basic idea with this fix is that if a string is not valid in the currnet
charset, then (unless it is byte-equivilant) it cannot be case-equivilant
to any other string.

This should address the majority of our smb_panic() cases on this matter.  It
will not fix them all - we still call unix_strupper(), aka strupper_m()
elsewhere, but this was being called on every file in the directory when
we performed unix_convert().

Tested with the stf unit tests for this routine.

Andrew Bartlett