Do not crash ldap backend on keyless principals

Operations like "kadmin -q 'addprinc -nokey foo'" or
"kadmin -q 'purgekeys -all foo'" result in principal entries with
no keys present, so krb5_encode_krbsecretkey() would just return
NULL, which then got unconditionally dereferenced in
krb5_add_ber_mem_ldap_mod().

Apply some fixes to krb5_encode_krbsecretkey() to handle zero-key
principals better, correct the test for an allocation failure, and
slightly restructure the cleanup handler to be shorter and more
appropriate for the usage.  Once it no longer short-circuits when
n_key_data is zero, it will produce an array of length two with both
entries NULL, which is treated as an empty list by the LDAP library,
the correct behavior for a keyless principal.

However, attributes with empty values are only handled by the LDAP
library for Modify operations, not Add operations (which only get
a sequence of Attribute, with no operation field).  As such, only
add krbprincipalkey to the modlist when we will be performing a
Modify, and not when we will be performing an Add, which is conditional
on the (misspelled) create_standalone_prinicipal boolean.

The purgekeys -all feature appears to have never worked with the
LDAP backend since its introduction in release 1.12.

ticket: 8041 (new)
tags: pullup
target_version: 1.13.1
subject: kadmind with ldap backend crashes when putting keyless entries
Regression tests for keyless principals

Confirm that kadmind does not crash when creating/modifying a principal
to have no keys, and confirm that no keys are present after a
purgekeys -all.
Remove some dead code

The secretkey variable is initialized to NULL and compared against
NULL, but never actually set to anything after initialization.

Remove the variable and all code that would have executed if it
was non-NULL.
Add helper for freeing arrays of berval pointers

This eliminates a potential leak of the bv_val members from
krb5_encode_krbsecretkey().
Correct spelling

Remove extra 'i' from "create_standalone_prinicipal".  While here,
pick a slightly shorter name for the variable.