units: introduce new Documentation= field and make use of it everywhere

This should help making the boot process a bit easier to explore and
understand for the administrator. The simple idea is that "systemctl
status" now shows a link to documentation alongside the other status and
decriptionary information of a service.

This patch adds the necessary fields to all our shipped units if we have
proper documentation for them.

src/shared/util.c

@@ -5610,3 +5610,24 @@ int can_sleep(const char *type) {
         free(p);
         return found;
 }
+
+bool is_valid_documentation_url(const char *url) {
+        assert(url);
+
+        if (startswith(url, "http://") && url[7])
+                return true;
+
+        if (startswith(url, "https://") && url[8])
+                return true;
+
+        if (startswith(url, "file:") && url[5])
+                return true;
+
+        if (startswith(url, "info:") && url[5])
+                return true;
+
+        if (startswith(url, "man:") && url[4])
+                return true;
+
+        return false;
+}

src/shared/util.h

@@ -508,4 +508,6 @@ int getenv_for_pid(pid_t pid, const char *field, char **_value);
 
 int can_sleep(const char *type);
 
+bool is_valid_documentation_url(const char *url);
+
 #endif