Q213-024 why3server allows to take long path as socket

Unix sockets have the restriction that the path to the socket must be
quite short (100 chars give or take depending on exact OS). We can
workaround this limitation by chdir'ing to the dirname of the socket
first. We need to do that in the client and server parts of the code,
but only for Unix (windows doesn't use Unix sockets obviously).

Right now this code is not strictly needed in gnatprove, because we
don't pass a path, only a filename as the socket. But this will change
later.

* prove_client.ml
(client_connect): save curdir, change to dirname, connect to basename,
  then go back to curdir
* options.c
rename socketname because it would clash with basename from libgen.h
* server-unix.c
rename of basename to socketname
(server_init_listening): save curdir, change to dirname, connect to
  basename, then go back to curdir
* server-win.c
rename of basename to socketname

Change-Id: Ib69587dfa136f6ced753c983bc37203c37acb936
(cherry picked from commit e6c15eb26d6077e814ab8c72deb92fc86e43600a)