Use void arguments list consistently in C code

On my way, fixed a couple of cases where we had `()` in the header, and
non-empty argument list in the source file.

PUBLISHED_FROM=5519526cf84e2bbd425a726fcc112fea1a95cbf1

docs/c-api/net.h/mg_time.md

@@ -3,7 +3,7 @@ title: "mg_time()"
 decl_name: "mg_time"
 symbol_kind: "func"
 signature: |
-  double mg_time();
+  double mg_time(void);
 ---
 
 A sub-second precision version of time().