Convert success in krb5_chpw_result_code_string

Result code 0 used to be converted properly by krb5_set_password,
though not krb5_change_password; this changed in 1.10 when
krb5int_setpw_result_code_string was folded into
krb5_chpw_result_code_string.  Restore the old behavior, and make it
apply to krb5_change_password as well, by making
krb5_chpw_result_code_string convert result code 0.

[ghudson@mit.edu: commit message]

ticket: 7569 (new)
target_version: 1.11.1
tags: pullup