Fix a variety of one-time leaks

Fix a variety of one-time leaks

Eliminate some memory leaks which should not affect normal operation,
but which make it harder to detect more serious memory leaks.

In kdb5_util, start using the already existing quit() function and
remove redundant DB and master key cleanup performed by individual
commands.  In kdb5_destroy(), use util_context instead of creating a
new one.  Add an mkey_fullname global variable and use it to make
a bunch of krb5_db_setup_mkey_name() calls unnecessary.