Add shims for mongoose file operations

PUBLISHED_FROM=b5b12cdf4273f21dbcdac9bfb6a06063152914c9

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

@@ -8,6 +8,8 @@ items:
   - { name: mg_basic_auth_header.md }
   - { name: mg_conn_addr_to_str.md }
   - { name: mg_fopen.md }
+  - { name: mg_fread.md }
+  - { name: mg_fwrite.md }
   - { name: mg_hexdump.md }
   - { name: mg_hexdump_connection.md }
   - { name: mg_hexdumpf.md }

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

@@ -0,0 +1,12 @@
+---
+title: "mg_fread()"
+decl_name: "mg_fread"
+symbol_kind: "func"
+signature: |
+  size_t mg_fread(void *ptr, size_t size, size_t count, FILE *f);
+---
+
+Reads data from the given file stream.
+
+Return value is a number of bytes readen. 
+

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

@@ -0,0 +1,12 @@
+---
+title: "mg_fwrite()"
+decl_name: "mg_fwrite"
+symbol_kind: "func"
+signature: |
+  size_t mg_fwrite(const void *ptr, size_t size, size_t count, FILE *f);
+---
+
+Writes data to the given file stream.
+
+Return value is a number of bytes wtitten. 
+