- Silence warnings: apply `const' generously, mark some variables

- Silence warnings: apply `const' generously, mark some variables
  __unused, and change local variables named `sin' (struct
  sockaddr_in) to `sin4'.  (`sin' conflicts with the definition of
  sin(3), which gcc assumes to be defined even if math.h isn't
  included (it's a builtin).  This is probably a bug in gcc.)

- Apply WARNS=1.  WARNS=2 was not used because this program assigns
  string literals to (struct iovec).iov_base for writing, and the only
  clean way to silence -Wwrite-strings in that case would be to
  strdup() and consequently free() those literals, which I considered
  too disruptive.

Reviewed by:	bde (partially)