srs{ue,enb,epc}: Disable pcap by default, allow enabling by param

Take the change to fix several small things and support recording pcap
in srsENB.

pcap generation can be enabled with scenario cfg-srs-enable-pcap.

Change-Id: Ia096a9be7efb2123f95115c751e2402fb4fec935
diff --git a/example/defaults.conf b/example/defaults.conf
index 9b92ad0..60539c8 100644
--- a/example/defaults.conf
+++ b/example/defaults.conf
@@ -97,9 +97,14 @@
   mcc: 901
   mnc: 70
   rlc_drb_mode: UM
+  enable_pcap: false
 
 srsenb:
   mcc: 901
   mnc: 70
   num_prb: 100
   transmission_mode: 1
+  enable_pcap: false
+
+srsue:
+  enable_pcap: false