fix for potential read builtin crash if bash gets a signal before read reads any input; make wait -f work if there are no pid or job arguments and update documentation; save and restore variable reflected in $BASH_COMMAND around shell function calls

tests/trap2.sub

@@ -50,13 +50,8 @@ exit 42 | command false
 command command command false
 
 unset FALSE
-if [ -x /bin/false ]; then
-        FALSE=/bin/false
-elif [ -x /usr/bin/false ]; then
-        FALSE=/usr/bin/false
-elif [ -x /usr/local/bin/false ]; then
-	FALSE=/usr/local/bin/false
-else
+FALSE=$(type -P false)
+if [ -z "$FALSE" ]; then
         FALSE='command false'
 fi