unsigned rc makes every send-error check in the list handlers dead code

[mgrossmann]

Found while working on #249, deliberately left alone there to keep that PR to
one subject.

The defect

datasetListHandler() and memberListHandler() both declare:

unsigned	rc		= 0;

and then check every write like this:

if ((rc = http_printf(session->httpc, "...")) < 0) goto quit;

rc is unsigned, so rc < 0 is never true. Every one of those checks is dead
code — a failed http_printf() or http_resp() is assigned and discarded,
and the handler keeps writing to a socket that is already gone. A client that
disconnects mid-listing has the whole remaining directory written after it.

It compiles clean because -Wall does not include -Wtype-limits
(that is -Wextra).

Where it is

• src/dsapi.c — datasetListHandler()
• src/dsapi.c — memberListHandler()

Worth grepping the other handlers in the same pass: the pattern is copied
around, and anywhere rc is unsigned the same checks are inert.

Note on the fix

Changing the declaration to int is a one-word edit but it is not a no-op —
it activates every one of those goto quit paths for the first time. The
cleanup at quit: needs a read in that light before flipping it.

For what it is worth, an early exit there appears harmless today: handle_request()
returns the handler's rc to mvsmf.c, which assigns it to irc and returns 0
regardless, so nothing tries to send a 500 on top of a partly written body. The
headers_sent guard in router.c:157 only covers the abend path.

member_scan() (added in #265) is currently the only place in the member list
where a write failure is actually detected, because it returns int.