Harden the popenv.sh scripts against unset variables

[sfkleach]

Issue #132 was caused by auto-generated popenv.sh scripts not warning about unset variables. Adding set -eu at the start of these scripts still allows them to be sh vs bash agnostic but ought to make them safer to use.

[sfkleach]

This boils down to adding a single line set -eu into makePopEnv.sh

cat  > "$popenv" <<\****
set -eu
pop_pop11="-$popsavelib/startup.psv"; export pop_pop11
pop_prolog="$pop_pop11 -$popsavelib/prolog.psv"; export pop_prolog
pop_clisp="$pop_pop11 -$popsavelib/clisp.psv"; export pop_clisp
pop_pml="$pop_pop11 +$popsavelib/pml.psv"; export pop_pml
pop_ved="$pop_pop11 :sysinitcomp();ved"; export pop_ved
****