dbi utils: Don't keep invalid object address after closing DBI connections.
sysdb: Check for and use libedit/libreadline for command input.
sysdb: Don't warn about missing history file when loading history.
sysdb: Free memory allocated by readline().
strbuf utils: Added sdb_strbuf_clear().
strbuf_test: Added a couple of tests operating on a zero-size buffer.

This is mostly about checking that this does not crash.
strbuf_test: Added tests for sdb_strbuf_clear().