error: New macro ERRP_GUARD()

Introduce a new ERRP_GUARD() macro, to be used at start of functions
with an errp OUT parameter.

It has three goals:

1. Fix issue with error_fatal and error_prepend/error_append_hint: the
user can't see this additional information, because exit() happens in
error_setg earlier than information is added. [Reported by Greg Kurz]

2. Fix issue with error_abort and error_propagate: when we wrap
error_abort by local_err+error_propagate, the resulting coredump will
refer to error_propagate and not to the place where error happened.
(the macro itself doesn't fix the issue, but it allows us to [3.] drop
the local_err+error_propagate pattern, which will definitely fix the
issue) [Reported by Kevin Wolf]

3. Drop local_err+error_propagate pattern, which is used to workaround
void functions with errp parameter, when caller wants to know resulting
status. (Note: actually these functions could be merely updated to
return int error code).

To achieve these goals, later patches will add invocations
of this macro at the start of functions with either use
error_prepend/error_append_hint (solving 1) or which use
local_err+error_propagate to check errors, switching those
functions to use *errp instead (solving 2 and 3).

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Reviewed-by: Paul Durrant <paul@xen.org>
Reviewed-by: Greg Kurz <groug@kaod.org>
Reviewed-by: Eric Blake <eblake@redhat.com>
[Merge comments properly with recent commit "error: Document Error API
usage rules", and edit for clarity.  Put ERRP_AUTO_PROPAGATE() before
its helpers, and touch up style.  Tweak commit message.]
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20200707165037.1026246-2-armbru@redhat.com>
[Rename ERRP_AUTO_PROPAGATE() to ERRP_GUARD(), tweak commit message
again]