Fix bug in strbuf_puts(), not always appending nul
Add 'rhizome.clean_on_open' config option
Add 'rhizome delete' and 'rhizome clean' commands

sqlite_void_exec() and its ilk now return the count of changed rows, not
just zero, on success

sqlite_exec_prepared() and its ilk now return the count of rows (number
of step results SQLITE_ROW), instead of just zero, on success

rhizome_clean() function now produces an optional report of its changes

rhizome_fail_write() and rhizome_finish_write() now log WARNings not
ERRORs if the SQL DELETE FROM FILES or DELETE FROM FILEBLOBS statements
fail

Refactor rhizome_open_read() to use sqlite_exec_int64()

Ensure that 'rhizome extract' and 'rhizome dump' commands return exit
status of 1 in "not found" conditions, not 255, which is reserved for
errors

Test cases for four new commands: 'rhizome delete bundle', 'rhizome
delete manifest', 'rhizome delete payload' and 'rhizome delete file'
(no test case for 'rhizome clean' yet)