maint: add shebang

This commit adds a test called `shebang`, which checks if all shebangs
in scripts use relative paths through `#/usr/bin/env`, rather than
absolute paths, such as `#/bin/bash`.

See 833b10575b.

maint/shebang

@@ -0,0 +1,10 @@
+#!/usr/bin/env bash
+
+if git grep -rnI "^#\!/" :/ | grep -v "#\!/usr/bin/env"
+then
+	echo "Absolute shebangs found, replace them with \"#!/usr/bin/env\""
+	exit 1
+else
+	echo "Everything seems ok"
+	exit 0
+fi