Add mg_hexdumpf: mg_hexdump that outputs to a file

PUBLISHED_FROM=f0fe58c9f01ef0c7b491ed0e5f51b983e4119507

docs/c-api/util.h/intro.md

@@ -10,6 +10,7 @@ items:
   - { name: mg_fopen.md }
   - { name: mg_hexdump.md }
   - { name: mg_hexdump_connection.md }
+  - { name: mg_hexdumpf.md }
   - { name: mg_is_big_endian.md }
   - { name: mg_match_prefix.md }
   - { name: mg_mbuf_append_base64.md }

docs/c-api/util.h/mg_hexdumpf.md

@@ -0,0 +1,10 @@
+---
+title: "mg_hexdumpf()"
+decl_name: "mg_hexdumpf"
+symbol_kind: "func"
+signature: |
+  void mg_hexdumpf(FILE *fp, const void *buf, int len);
+---
+
+Same as mg_hexdump, but with output going to file instead of a buffer. 
+