Make bash more strict

[cjgillot]

Bash has a lot of implicit semantics for non-existent variables.
This can cause competing directives (like $OMP) to be eaten silently.

Setting a stricter mode for bash by default can help a lot.

set -e # Abort on first error
set -u # Err on undefined variable
set -o pipefail # Err on redirection failure

This would be a breaking change. The zpp standard library is not set -u safe.
See gysela merge !633 for a few examples.

[jbigot]

I fully agree with set -u and -o pipefail. Regarding -e however, this breaks some perfectly valid patterns like:

dosomething
if $?; then ...