Enforce *no networking* in single-user mode, runlevel 1

This is a quite intrusive change.  In addition to the already reserved
runlevels 0 and 6, halt and reboot, respectively, this patch reserves
runlevel 1 as *no networking*, like the traditional SysV init did.  One
of the reasons for this change is the new 'single' user mode, introduced
earlier.  Most users when entering 'S' or 'single' on the kernel cmdline
expect the single user mode (runlevel 1) to be without networking.

This leaves 2, 3, 4, 5, and 7, 8, 9 as fully user-configurable runlevels
with networking.

Of course, networking can still be enabled in runlevel 1 using ifconfig
or ifupdown, or change runlevel.

Signed-off-by: Joachim Nilsson <troglobit@gmail.com>

src/helpers.h

@@ -68,6 +68,7 @@ int     print_result    (int fail);
 int     getuser         (char *username, char **home);
 int     getgroup        (char *group);
 void    set_hostname    (char **hostname);
+void    networking      (int updown);
 
 int     complete        (char *cmd, int pid);
 int     run             (char *cmd);