Correctly get default realm in kdb5.c

Call krb5_get_default_realm instead of directly accessing
context->default_realm, to remove the requirement that
krb5_get_default_realm or krb5_set_default_realm be used before
krb5_db_open.
Fix SAM-2 preauth when password argument is used

sam2_process accesses gak_data to get the password, so that it can do
string-to-key with the etype in the SAM-2 challenge.  When #7642
changed gic_pwd.c to use struct gak_password instead of krb5_data,
sam2_process wasn't altered to match.  We don't see a problem when the
password is read through the prompter (as with kinit), because the
password winds up in the storage field at the beginning of the
gak_password structure.  But when a password is supplied as a
parameter (as with ksu), the storage field is empty and we get the
wrong answer from sam2_process.

ticket: 7862
target_version: 1.12.2
tags: pullup
Don't use system dictionary files for DB2 tests

The system dictionary may contain entries with punctuation, which can
confuse the shell.  It's more predictable to always use the word list
from the source tree.

ticket: 7860
status: open
Generate dependencies for t_stringattr

When t_stringattr.c was introduced, it wasn't added to EXTRADEPSRCS,
so no dependencies were created for it.  Fix that now.
Update doc build instructions

Documentation build instructions in doc/README were out of date.
Update them.

ticket: 7864 (new)
tags: pullup
target_version: 1.12.2
Don't leak those creds