accept(),accept4(): Don't set *addrlen = 0 on [ECONNABORTED].

If the underlying protocol reported an error (e.g. because a connection was
closed while waiting in the queue), this error was also indicated by
returning a zero-length address. For all other kinds of errors (e.g.
[EAGAIN], [ENFILE], [EMFILE]), *addrlen is unmodified and there are
successful cases where a zero-length address is returned (e.g. a connection
from an unbound Unix-domain socket), so this error indication is not
reliable.

As reported in Austin Group bug #836, modifying *addrlen on error may cause
subtle bugs if applications retry the call without resetting *addrlen.