examples: execute bash sessions in script environment

Previously, the sessions contained only explicitly exported variables
and functions.
This was fragile and in part buggy due to lacking exports.

Interactive features like user-defined aliases and functions are still
working as before.

examples/start-bash-session.sh

@@ -0,0 +1,6 @@
+# Start an interactive bash session in the current bash environment.
+
+# BASH_ENVIRONMENT contains definitions of read-only variables like 'BASHOPTS' that
+# cause warnings on importing. Suppress these warnings during bash startup.
+BASH_ENVIRONMENT=<(declare -p; declare -pf) \
+  bash --rcfile <(echo 'source $BASH_ENVIRONMENT 2>/dev/null')