bug: Use en_US when fetching EPOCHREALTIME

Isolates fetching of EPOCHREALTIME to a function which sets LC_ALL=en_US.UTF-8.
This ensures that the value is in decimal format, regardless of runtime locale.

bug: Hide duration when no command executed

plugins/available/cmd-returned-notify.plugin.bash

@@ -4,7 +4,8 @@ about-plugin 'Alert (BEL) when process ends after a threshold of seconds'
 
 function precmd_return_notification() {
 	local command_start="${COMMAND_DURATION_START_SECONDS:=0}"
-	local current_time="${EPOCHREALTIME:-$SECONDS}"
+	local current_time
+	current_time="$(_shell_duration_en)"
 	local -i command_duration="$((${current_time%.*} - ${command_start%.*}))"
 	if [[ "${command_duration}" -gt "${NOTIFY_IF_COMMAND_RETURNS_AFTER:-5}" ]]; then
 		printf '\a'