Add scsh-collect-ports-interface to scsh-level-0.
Fix (begin ...) process form during redirections.

Use scsh's %dup2 instead of s48's dup2. The latter closes any ports opened on
the corresponding fd. Unfortunately, this includes (current-*-port) and we're
unable to set those to arbitrary ports for the process globally, whereas posix
dup2 is a process global operation.