socket-util: slight rework of getpeersec()

socket-util: slight rework of getpeersec()

Let's call getsockopt() in a loop, so that we can deal correctly with
the label changing while we are trying to read it.

(also, while we are at it, let's make sure that there's always one
trailing NUL byte at the end of the buffer, after all SO_PEERSEC has
zero documentation, and multiple implementing backends, hence let's
better be safe than sorry)