Convert Samba3 to use the common lib/util/charset API

Convert Samba3 to use the common lib/util/charset API

This removes calls to push_*_allocate() and pull_*_allocate(), as well
as convert_string_allocate, as they are not in the common API

To allow transition to a common charcnv in future, provide Samba4-like
strupper functions in source3/lib/charcnv.c

(the actual implementation remains distinct, but the API is now shared)

Andrew Bartlett