Take advantage of base64 conversion in testapps

There was a non-working function and a working one in the source of
the contributed "rtp_decoder".  Have a working conversion function
and use it from both tools for parsing the command-line crypto-key.
diff --git a/README b/README
index 97f382c..b3ee608 100644
--- a/README
+++ b/README
@@ -78,7 +78,7 @@
   Manual srtp keying uses the -k option; automated key management
   using gdoi will be added later.
 
-usage: rtpw [-d <debug>]* [-k <key> [-a][-e <key size>][-g]] [-s | -r] dest_ip dest_port
+usage: rtpw [-d <debug>]* [-k|b <key> [-a][-e <key size>][-g]] [-s | -r] dest_ip dest_port
 or     rtpw -l
 
   Either the -s (sender) or -r (receiver) option must be chosen.
@@ -96,6 +96,8 @@
 		key is a hexadecimal value (without the
                 leading "0x")
 
+  -b <key>      same as -k but with base64 encoded key
+
   -e <keysize>  encrypt/decrypt (for data confidentiality)
                 (requires use of -k option as well)
                 (use 128, 192, or 256 for keysize)