Add mg_ prefix to mongoose sources

CL: none

PUBLISHED_FROM=d6ebe5fa88c61ae3b8569897d9b09d54610bec73

docs/c-api/mbuf.h/struct_mbuf.md

@@ -1,14 +0,0 @@
----
-title: "struct mbuf"
-decl_name: "struct mbuf"
-symbol_kind: "struct"
-signature: |
-  struct mbuf {
-    char *buf;   /* Buffer pointer */
-    size_t len;  /* Data length. Data is located between offset 0 and len. */
-    size_t size; /* Buffer size allocated by realloc(1). Must be >= len */
-  };
----
-
-Memory buffer descriptor 
-