re-add the configurable --with-privsep-path install-time sanity check

While this still doesn't affect the behavior of the daemon, the
configuration option can at least be set to check the correct privsep
directory for permissions. Revisit in 5.8 as a possible extension to the
runtime check instead to remove the 'knob'.

INSTALL

@@ -64,8 +64,8 @@ On most Linux and BSD systems, something like should work:
  chmod 0755 /var/empty
 
 /var/empty here is a chroot directory used by ntpd for privilege separation of
-the DNS and NTP processes. This directory should be completely empty, owned by
-root, and must not be group or world-writable.
+the DNS and NTP processes. This directory should not contain any files, must be
+owned by root, and must not be group or world-writable.
 
 NOTE:
 If you installed a previous OpenNTPD release and created a /var/empty/ntp
@@ -90,13 +90,17 @@ As of OS X 10.10, something like this should work similarly
  dseditgroup -o create _ntp
  dscl . append /Groups/_ntp GroupMembership _ntp
 
-There are few options to the configure script in addition to the ones
+There are a few options to the configure script in addition to the ones
 provided by autoconf itself:
 
 --with-privsep-user=[user]
 	Specify unprivileged user used for privilege separation.  The default
-	is "_ntp". The home directory of this user will be used for privilege
-	separation.
+	is "_ntp".
+
+--with-privsep-path=path
+	ntpd will always use the home directory of the privsep user
+	to chroot to, but specifying this parameter will change the
+	post-installation checks and instructions to match the specified path.
 
 --with-cacert=[path]
 	Specify the CA certificate location for HTTPS constraint validation.

configure.ac

@@ -230,7 +230,13 @@ AC_ARG_WITH(cacert,
 )
 AC_SUBST(CONSTRAINT_CA)
 
-PRIVSEP_PATH=/var/empty
+AC_ARG_WITH(privsep-path,
+	[  --with-privsep-path=path Specify privilege separation chroot path],
+	[ AC_DEFINE_UNQUOTED(NTPD_CHROOT_DIR, "$withval",
+	    [Privilege separation chroot path])
+	  PRIVSEP_PATH=$withval ],
+	[ PRIVSEP_PATH=/var/empty ]
+)
 AC_SUBST(PRIVSEP_PATH)
 
 AC_CONFIG_FILES([