Add the program “pipe”

Add the program “pipe”

The program “pipe” spawns a process for each argument,
except the last, which reuses the parent process.
All these processes are connected by pipes
such that the process i reads from i−1 and writes into i+1.
The first process has its stdin not redirected;
the last process has its stdout not redirected.
Each process execv()s into minish-eval,
passing the corresponding argument to it.
“pipe a b c” does what “a | b | c” does in sh,
except that, with pipe,
the process for “a” and for “b” are children of the process for “c”,
that is a child of the shell;
in “a | b | c“, all three process are children of the shell.