compiler: -Werror=return-type hardening

compiler: -Werror=return-type hardening

Warn whenever a function is defined with a return type that defaults to
int. Also warn about any return statement with no return value in a
function whose return type is not void (falling off the end of the
function body is considered returning without a value).

For C only, warn about a return statement with an expression in a
function whose return type is void, unless the expression type is also
void. As a GNU extension, the latter case is accepted without a warning
unless -Wpedantic is used.

Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>