lib/util.sh: removed no longer used limit_stderr_extern. use `limit_stderr_cmd extern` instead.

lib/util.sh: specify limit_stderr_cmd just for the (few) commands that need it, rather than for all commands, so that commands that use stderr to print important error messages don't have those error messages hidden when the verbosity is too low. (error messages should always be displayed, regardless of the verbosity.)

lib/util.sh: limit_stderr_cmd (): only echo the command if it starts with echo_run. this requires adding echo_run before commands that use limit_stderr_cmd, such as limit_stderr_extern.

lib/util.sh: limit_stderr_cmd (): remove echo_run from the command to run so that the command name isn't echoed twice

lib/util.sh: limit_stderr_cmd: alias-expand command after it

lib/util.sh: $verbosity: ensure it's an integer using `declare -i`

lib/util.sh: grouped set verbosity statements together and commented them

bugfix: lib/util.sh: verbose output: $verbosity defaults to $verbose (boolean) converted to integer. the previous set-default of $verbosity to $verbose has been removed because it came after `: "${verbosity=3}"` and thus didn't have an effect.

lib/util.sh: added bool2int ()

*{.sh,run}: use new isset