reftable: don't second-guess errors from flock interface

reftable: don't second-guess errors from flock interface

The `flock` interface is implemented as part of "reftable/system.c" and
thus needs to be implemented by the integrator between the reftable
library and its parent code base. As such, we cannot rely on any
specific implementation thereof.

Regardless of that, users of the `flock` subsystem rely on `errno` being
set to specific values. This is fragile and not documented anywhere and
doesn't really make for a good interface.

Refactor the code so that the implementations themselves are expected to
return reftable-specific error codes. Our implementation of the `flock`
subsystem already knows to do this for all error paths except one.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>