Make clang-analyze happy in two irrelevant cases.
plugin: Added sdb_plugin_shutdown_all().

This function calls all registered "shutdown" callbacks.
sysdbd: Shut down all plugins before exiting.
sysdbd: Free up the config item returned from the parser as well.

oconfig_free() does not do that because it does not correctly keep track of
what the top-level config item is and it cannot free items which are the child
of another.
frontend: Free strdup()'ed username when destroying a connection.