Convert Samba3 to use the common lib/util/charset code (partial)

Convert Samba3 to use the common lib/util/charset code (partial)

This parital commit has it's focus on converting Samba3 code to use
the common functions where possible, and exposes some of the
previously static common functions for use by what remains in
libsmb/charcnv.c

libsmb/charcnv.c contains the Samba-3 only wrapper functions still in use.

Andrew Bartlett