This was unnecessarily confusing. When we build up a chain of commands
connected by pipes, we now store the file descriptors of each end of
these pipes as rewirings in a vector. The rewirings are then put into
effect by calls to dup2().
If I'm understanding the standard C library correctly, setenv() copies while
putenv() does not. That's really confusing and putenv() basically sucks.
To know which environment variables to free on replacement and which ones to
leave alone, we keep track of the ones malloced by setenv in a side table.
This patch also moves Shell to using setenv() instead of putenv().
Fixes#29.
This is in keeping with how putenv should function. It does mean that
the shell's export command now leaks, but that's not a difficult fix.
Contributes to #29.
