Add alias support

Add alias support

Add a new kadmin command add_alias.  Implement it for DB2 and LMDB by
writing stub principal entries with a tl-data entry giving the target
name.  Add libkdb5 functions to create and interpret alias entries.
Handle these stub entries in krb5_db_get_principal(), iteratively
resolving aliases up to a depth of 10.

To allow kadm5_delete_principal() to work on aliases, remove the code
that fetches the entry prior to deletion; it was needed before commit
0780e46fc13dbafa177525164997cd204cc50b51 to decrement the policy
reference count, but now serves no purpose.  Adjust kdb_delete_entry()
to translate KRB5_KDB_NOENTRY instead of ignoring it, as we still want
to return KADM5_UNK_PRINC when deleting a nonexistent principal name.

Modify the LDAP KDB module to work with alias entries.  In
krb5_ldap_put_principal(), recognize stub alias entries and add an
alias to the object for the target principal.  In
krb5_ldap_delete_principal(), don't delete the LDAP object when
deleting an alias name.  In krb5_ldap_iterate(), generate stub entries
for each alias name in addition to the populated entry for the
canonical name.  A small amount of refactoring was done as part of
this work: the LDAP-specific principal name parsing and unparsing
functions were simplified, and a helper function search_princ() was
added to find the LDAP object for a principal name.

In kdb5_util tabdump, add a dump type "alias" to display a list of
aliases in the database.

Based on work by Alexander Bokovoy.