treewide: use modern syntax

treewide: use modern syntax

 - Use `||=` operator expressions instead of `expr = expr || value` patterns
 - Use optional chaining instead of `foo && foo.bar && foo.bar.baz` patterns
 - Use template strings instead of string concatenations or sprintf() calls
 - Use `const` for global default variables

Signed-off-by: Jo-Philipp Wich <jo@mein.io>