server: Simplify afs socket cookie code.

server: Simplify afs socket cookie code.

Currently, the afs_socket_cookie value is passed to afs_init() and
is stored in the commmand task structure of the afs process. This
is unnecessary since the variable defined in server.c is non-static,
and declared as extern in afs.c. We may thus refer to this variable
from afs.c.

The variable was also documented twice. Get rid of one comment.