Experimental #saveoptions command

Add a #saveoptions extended command, to allow saving configuration
settings from within the game. This is still highly experimental,
and gives plenty of warnings before asking to overwrite the file.

Lack of option saving is one of the biggest complaints new players
have, so this should help with it.  More experienced players with
highly customized config file should not use this feature, as it
completely rewrites the file, removing all comments and non-config
lines.

src/files.c

@@ -1984,6 +1984,48 @@ char configfile[BUFSZ];
 const char *backward_compat_configfile = "nethack.cnf";
 #endif
 
+/* #saveoptions - save config options into file */
+int
+do_write_config_file(void)
+{
+    FILE *fp;
+    char tmp[BUFSZ];
+
+    if (!configfile[0]) {
+        pline("Strange, could not figure out config file name.");
+        return ECMD_OK;
+    }
+    if (flags.suppress_alert < FEATURE_NOTICE_VER(3,7,0)) {
+        pline("Warning: saveoptions is highly experimental!");
+        wait_synch();
+        pline("Some settings are not saved!");
+        wait_synch();
+        pline("All manual customization and comments are removed from the file!");
+        wait_synch();
+    }
+#define overwrite_prompt "Overwrite config file %.*s?"
+    Sprintf(tmp, overwrite_prompt, (int)(BUFSZ - sizeof overwrite_prompt - 2), configfile);
+#undef overwrite_prompt
+    if (!paranoid_query(TRUE, tmp))
+        return ECMD_OK;
+
+    fp = fopen(configfile, "w");
+    if (fp) {
+        size_t len, wrote;
+        strbuf_t buf;
+
+        strbuf_init(&buf);
+        all_options_strbuf(&buf);
+        len = strlen(buf.str);
+        wrote = fwrite(buf.str, 1, len, fp);
+        fclose(fp);
+        strbuf_empty(&buf);
+        if (wrote != len)
+            pline("An error occurred, wrote only partial data (%lu/%lu).", wrote, len);
+    }
+    return ECMD_OK;
+}
+
 /* remember the name of the file we're accessing;
    if may be used in option reject messages */
 static void