Expose digest auth checking functions

PUBLISHED_FROM=1bfc6e332f56b68eb6155bb729a97a0d8d5a316c

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

@@ -5,6 +5,8 @@ decl_name: "http.h"
 items:
   - { name: mg_connect_ws.md }
   - { name: mg_connect_ws_opt.md }
+  - { name: mg_http_is_authorized.md }
+  - { name: mg_http_send_digest_auth_request.md }
   - { name: mg_printf_websocket_frame.md }
   - { name: mg_send_websocket_frame.md }
   - { name: mg_send_websocket_framev.md }

docs/c-api/http.h/mg_http_is_authorized.md

@@ -0,0 +1,16 @@
+---
+title: "mg_http_is_authorized()"
+decl_name: "mg_http_is_authorized"
+symbol_kind: "func"
+signature: |
+  int mg_http_is_authorized(struct http_message *hm, struct mg_str path,
+                            int is_directory, const char *domain,
+                            const char *passwords_file, int is_global_pass_file);
+---
+
+Checks whether an http request is authorized. `domain` is the authentication
+realm, `passwords_file` is a htdigest file (can be created e.g. with
+`htdigest` utility). If either `domain` or `passwords_file` is NULL, this
+function always returns 1; otherwise checks the authentication in the
+http request and returns 1 only if there is a match; 0 otherwise. 
+

docs/c-api/http.h/mg_http_send_digest_auth_request.md

@@ -0,0 +1,11 @@
+---
+title: "mg_http_send_digest_auth_request()"
+decl_name: "mg_http_send_digest_auth_request"
+symbol_kind: "func"
+signature: |
+  void mg_http_send_digest_auth_request(struct mg_connection *c,
+                                        const char *domain);
+---
+
+Sends 401 Unauthorized response. 
+