Do not catch SIGQUIT

So that it can be used to make a core dump during manual debugging
Improve Rhizome list cursor

Separate function rhizome_list_commit() which, if not called, causes the
next list re-query to include the last row that was fetched with
rhizome_list_next(), rather than excluding it.
Improve HTTP server generated content logic

Content generator functions now take arguments describing the buffer
they are to fill, and respond with a struct containing the number of
bytes filled, and the number of free bytes needed before being called
again.

The HTTP response logic now fills buffer as much as possible before
calling write(2) by topping it up instead of waiting for it to be
completely emptied before generating more content.