poke: rename -f/--force option to -u/--unsolicited

The old name will still be accepted for some time,
but will print a deprecation warning if used.

This commit also improves --help output.

Change-Id: I2ebac82423a5276109f4b40b382312bd852419de
diff --git a/manpages/ndnpoke.rst b/manpages/ndnpoke.rst
index 7810a8c..5f704ee 100644
--- a/manpages/ndnpoke.rst
+++ b/manpages/ndnpoke.rst
@@ -4,7 +4,7 @@
 Synopsis
 --------
 
-**ndnpoke** [-h] [-f] [-F] [-x *freshness*] [-i *identity*\|\ -D] [-w *timeout*] [-v] [-V] *name*
+**ndnpoke** [-h] [-u] [-F] [-x *freshness*] [-i *identity*\|\ -D] [-w *timeout*] [-v] [-V] *name*
 
 Description
 -----------
@@ -12,7 +12,7 @@
 :program:`ndnpoke` is a simple producer program that reads a payload from the standard
 input and publishes it as a single Data packet. The Data packet is either sent as a
 response to an incoming Interest matching *name*, or immediately pushed to the local
-NDN forwarder as "unsolicited Data" if the **-f** flag is specified.
+NDN forwarder as "unsolicited Data" if the **-u** flag is specified.
 
 Options
 -------
@@ -20,7 +20,7 @@
 ``-h, --help``
   Print help and exit.
 
-``-f, --force``
+``-u, --unsolicited``
   Send the Data packet without waiting for an incoming Interest.
 
 ``-F, --final``