Support site-local KDC discovery via DNS

Support site-local KDC discovery via DNS

Add the sitename realm variable.  If set, service location via DNS
will be attempted using the site name as specified in [MS-ADTS]
6.3.2.3, falling back to regular discovery on failure.

[ghudson@mit.edu: made this strictly a realm variable; moved
k5_get_sitename() to locate_kdc.c and made it take a krb5_data input;
fixed a memory leak; corrected documentation changes; fleshed out
commit message]

ticket: 9124 (new)