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.
Also run it across the whole tree to get everything using the One True Style.
We don't yet run this in an automated fashion as it's a little slow, but
there is a snippet to do so in makeall.sh.
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.
Hook this up in Terminal so that the '\a' character generates a beep.
Finally emit an '\a' character in the shell line editing code when
backspacing at the start of the line.
