Add an ACL list for users connecting to the tmux socket. Users may be forbidden

from attaching, forced to attach read-only, or allowed to attach read-write. A
new command, server-access, configures the list. tmux gets the user using
getpeereid(3) of the client socket. Users must still configure file system
permissions manually.

Makefile.am

@@ -123,6 +123,7 @@ dist_tmux_SOURCES = \
 	cmd-select-pane.c \
 	cmd-select-window.c \
 	cmd-send-keys.c \
+	cmd-server-access.c \
 	cmd-set-buffer.c \
 	cmd-set-environment.c \
 	cmd-set-option.c \
@@ -172,6 +173,7 @@ dist_tmux_SOURCES = \
 	screen-redraw.c \
 	screen-write.c \
 	screen.c \
+	server-acl.c \
 	server-client.c \
 	server-fn.c \
 	server.c \