Document ifreload new option + add it into init.d networking script

Ticket:
Reviewed By: trivial
Testing Done: Tested build/install/boot + ifupdown2 sanity

init.d/networking

@@ -166,12 +166,21 @@ reload)
 	log_action_end_msg $?
 	;;
 
+reload-currently-up)
+
+	ifupdown_init
+	log_action_begin_msg "Reloading currently up network interfaces configuration"
+
+	ifreload --currently-up $EXTRA_ARGS
+	log_action_end_msg $?
+	;;
+
 force-reload)
 
 	ifupdown_init
 
 	log_action_begin_msg "Reloading network interfaces configuration"
-	ifreload -a
+	ifreload -f -a $EXTRA_ARGS
 	log_action_end_msg $?
 	;;
 

man.rst/ifreload.8.rst

@@ -14,7 +14,7 @@ reload network interface configuration
 
 SYNOPSIS
 ========
-    ifreload [-h] -a [-v] [-d] [-f] [-n] 
+    ifreload [-h] (-a|-c) [-v] [-d] [-f] [-n] 
 
 DESCRIPTION
 ===========
@@ -27,6 +27,10 @@ DESCRIPTION
     but it skips **ifdown** for interfaces that did not change in the config
     file.
 
+    If you do not wish to execute **down** on any interfaces, but only **up** on
+    interfaces that were already **up**, please see the **--currently-up**
+    option below.
+
 
 OPTIONS
 =======
@@ -40,6 +44,11 @@ OPTIONS
 
     -f, --force           force run all operations
 
+    -c, --currently-up    only reload auto and other interfaces that are
+                          currently up. This can be used as a non-disruptive
+                          alternative to -a because it will not down any
+                          interfaces
+
 EXAMPLES
 ========
     # reload all auto interfaces in **interfaces(5)** file
@@ -50,6 +59,10 @@ EXAMPLES
 
     **service networking reload**
 
+    # reload all currently up interfaces without bringing any interfaces down
+
+    **service networking reload-currently-up**
+
 SEE ALSO
 ========
     ifup(8),

sbin/ifupdown

@@ -353,6 +353,9 @@ def update_ifreload_argparser(argparser):
     argparser.add_argument('-l', '--syslog', dest='syslog',
                 action='store_true',
                 help=argparse.SUPPRESS)
+    argparser.add_argument('-f', '--force', dest='force',
+                action='store_true',
+                help='force run all operations')
 
 def parse_args(argsv, op):
     if op == 'query':