lib/util/charset Use push_string and talloc_strupper/strlower from common code

lib/util/charset Use push_string and talloc_strupper/strlower from common code

The only caller of push_string() (not to be confused with
push_string_check()) in the common code was encode_pw_buffer(), and it
didn't use the alignment or STR_UPPER flags.

The talloc_strupper() and talloc_strlower() functions are tested in
smbtorture, and are next_codepoint() based.

Andrew Bartlett