meson: use "sh -eu" and make .sh +x, .py -x

Shell scripts should be executable so that meson reports their
invocation succinctly (does not print 'sh' '-e').
Python scripts should not be executable so that meson does the
detection of the right python binary itself.

Add -u everywhere to catch potential errors.

src/boot/efi/no-undefined-symbols.sh

@@ -1,4 +1,4 @@
-#!/bin/sh -e
+#!/bin/sh -eu
 
 if nm -D -u "$1" | grep ' U '; then
     echo "Undefined symbols detected!"