ITS#6197 - normalize & speed up ldap_err2string/ldap_perror():

ITS#6197 - normalize & speed up ldap_err2string/ldap_perror():

Use same result code -> string mapping for both, removing ldap_int_error():
- Classify unknown error codes as API/extension/etc like _err2string did.
- Pass all strings through _(), i.e. the optional gettext, as _perror did.

Also use a switch instead of linear search for the code->string mapping.
Hopefully the compiler will optimize that.  Though the extra gettext,
if anyone uses it, probably counters that speedup.