Support running an executable file when a new lease is acquired.

If no 'script-file = SCRIPTFILE' is specified in the configuration
file and if no '-X SCRIPTFILE' or '--script-file SCRIPTFILE'
command argument is provided, then this functionality is entirely
inactive and no associated subprocess is spawned.

Otherwise, ndhc will spawn a subprocess that runs as root that has the
sole job of forking off a subprocess that exec's the specified script in
a sanitized and fixed-state environment whenever a new DHCPv4 lease is
acquired.

Note that this script is provided no information about ndhc or the
DHCP state in the environment or in any argument fields; it is the
responsibility of this script to gather whatever information it needs
from either the filesystem or syscalls.  This design is intended to
avoid the historical problems that are associated with dhcp clients
invoking scripts.

The path of the scriptfile cannot be changed after ndhc is initially
run; ndhc forks off the privsep script subprocess that executes scripts
after it has read the configuration file and command arguments, but
before it begins processing network data; thus, it is impossible for the
network-handling process to modify or influence the script assuming
proper OS memory protection.

The privsep channel communicates that the script should be run by simply
writing a newline; anything else will result in ndhc terminating itself.

Before the recommended way to update system state after a change in
lease information was to run the fcactus program and watch the
associated leasefile for the interface for modification; now no external
program is needed for this job.

ndhc.h

@@ -69,9 +69,12 @@ extern int ifchSock[2];
 extern int ifchStream[2];
 extern int sockdSock[2];
 extern int sockdStream[2];
+extern int scriptdSock[2];
+extern int scriptdStream[2];
 extern char state_dir[PATH_MAX];
 extern char chroot_dir[PATH_MAX];
 extern char resolv_conf_d[PATH_MAX];
+extern char script_file[PATH_MAX];
 extern uid_t ndhc_uid;
 extern gid_t ndhc_gid;