whatnow cooks up strings for executing external commands, and then

whatnow cooks up strings for executing external commands, and then
feeds these strings to popen or system.  These command strings rely on
$SHELL being present - and if that is not the case, then we get weird
error messages ("sh: -c not found" or similar).  (Because both
system() and popen() start up fine with the std shell, but their arg
string to parse doesn't include a cmdname/path before the -c.)

As far as i understand the POSIX standard, SHELL is recommended but
not actually mandatory, so i think it would be good to handle this a
bit more robustly: by setting SHELL to /bin/sh if not present.