lib/util/charset create _handle functions for convert_string() et al

lib/util/charset create _handle functions for convert_string() et al

This is now API compatible with the existing code in lib/util/charset

lazy_initialize_conv() is no longer called as init_iconv() is called
when the smb.conf is processed, and get_conv_handle() will auto-init
with defaults if required.

load_case_tables_library() is no longer requried as all binaries and
libraries already load these in their entry points, as otherwise all
the other string functions would fail.

Andrew Bartlett