It is probably best to base the functions on Unicode code points instead of
scalar values, now that they are also used from sal/rtl/strtmpl.cxx with UTF-16
code units and with arbitrary bytes (with values assumed to be a superset of
ASCII, though).  Rename compareAsciiIgnoreCase to compareIgnoreAsciiCase.  Also,
the corresponding tools::INetMIME functions can be removed completely; no need
to keep them around as deprecated.

Change-Id: I8d322177f4909e70a946e8186e3e0f7fa6d9a43e