Keep a reference to an owning user in each usage table. We want to allow
callers to hold charges without holding on to any user references.

Also fix the peer-deinitialization to be ordered correctly and free the
user references last (in particular, after the charges). This is not
strictly necessary, but now follows our coding style and would have
avoided possible failures.

This fixes an assertion failure when disconnecting entire groups of
peers of the same user, due to the recent fix that actually made
peer-accounting do something.

Reported-by: Torge Matthies
Reported-by: Mel34
Signed-off-by: David Rheinsberg <david.rheinsberg@gmail.com>

While service_free() correctly releases the strv in `argv`, the
service_update() path does not. It frees `argv`, but not the individual
entries. Fix this and properly release all entries.

Reported-by: Evgeny Vereshchagin
Signed-off-by: David Rheinsberg <david.rheinsberg@gmail.com>

Use the new c_mem*() functions rather than mem*() so we protect against
NULL pointers in empty areas, which are UB with the classic mem*()
functions.

Signed-off-by: David Rheinsberg <david.rheinsberg@gmail.com>

Fill in NEWS, update AUTHORS, and bump version to 31.

Signed-off-by: David Rheinsberg <david.rheinsberg@gmail.com>