stat_cache and strupper_m()

[Andrew Bartlett]

Jeremy,

I was looking over our strupper code users again, and in particular
your changes to the stat cache, to cope with expanding and 
contracting strings.

This patch reimplements strdup_upper() to always use 
convert_string_allocate(), and therefore always expand
and contract as required.

(rather than just contractions)

I've not tested this with anything by ascii, but I wanted to put 
the idea out for others to look at.  Also, I would note that 
stat_cache_add() currently asserts that the length of the two
strings is equal - I think it might need similar work to that
in stat_cache_lookup().

Andrew Bartlett