Adding "conf.d" configuration dir support.

Any file matching the pattern /etc/tinc/$NETNAME/conf.d/*.conf will be
parsed after the tinc.conf file.

doc/tinc.conf.5.in

@@ -668,6 +668,8 @@ The top directory for configuration files.
 .It Pa @sysconfdir@/tinc/ Ns Ar NETNAME Ns Pa /tinc.conf
 The default name of the server configuration file for net
 .Ar NETNAME .
+.It Pa @sysconfdir@/tinc/ Ns Ar NETNAME Ns Pa /conf.d/
+Optional directory from which any .conf file will be loaded
 .It Pa @sysconfdir@/tinc/ Ns Ar NETNAME Ns Pa /hosts/
 Host configuration files are kept in this directory.
 .It Pa @sysconfdir@/tinc/ Ns Ar NETNAME Ns Pa /tinc-up

doc/tinc.texi

@@ -793,6 +793,9 @@ The actual configuration of the daemon is done in the file
 @file{@value{sysconfdir}/tinc/@var{netname}/tinc.conf} and at least one other file in the directory
 @file{@value{sysconfdir}/tinc/@var{netname}/hosts/}.
 
+An optionnal directory @file{@value{sysconfdir}/tinc/@var{netname}/conf.d} can be added from which
+any .conf file will be read.
+
 These file consists of comments (lines started with a #) or assignments
 in the form of
 

src/conf.c

@@ -376,6 +376,29 @@ bool read_server_config(void) {
 	errno = 0;
 	x = read_config_file(config_tree, fname);
 
+	// We will try to read the conf files in the "conf.d" dir
+	if (x) {
+		char * dname;
+		xasprintf(&dname, "%s" SLASH "conf.d", confbase);
+		DIR *dir = opendir (dname);
+		// If we can find this dir
+		if (dir) { 
+			struct dirent *ep;
+			// We list all the files in it
+			while (x && (ep = readdir (dir))) {
+				size_t l = strlen(ep->d_name);
+				// And we try to read the ones that end with ".conf"
+				if (l > 5 && !strcmp(".conf", & ep->d_name[ l - 5 ])) {
+					free(fname);
+					xasprintf(&fname, "%s" SLASH "%s", dname, ep->d_name);
+					x = read_config_file(config_tree, fname);
+				}
+			}
+			closedir (dir);
+		}
+		free(dname);
+	}
+
 	if(!x && errno)
 		logger(DEBUG_ALWAYS, LOG_ERR, "Failed to read `%s': %s", fname, strerror(errno));