Refactor and audit which(1)

Use the *at functions instead of building paths manually. We do
still have path-building in recurse() and other areas, but the
long-term goal is to rid most interfaces of that for practical
and security reasons.
In this case, it's more or less trivial.

Also, refactor the manpage to be more consistent with the others.

BUGFIX: Return exit status 3 on error.

README

@@ -91,7 +91,7 @@ The following tools are implemented:
 =*|o uudecode    .
 =*|o uuencode    .
 #*|o wc          .
-=* x which       .
+=*|x which       .
 =*|o xargs       (-p)
 =*|x yes         .