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/scenarios/cfg-srs-enable-pcap.conf b/example/scenarios/cfg-srs-enable-pcap.conf
new file mode 100644
index 0000000..6927b80
--- /dev/null
+++ b/example/scenarios/cfg-srs-enable-pcap.conf
@@ -0,0 +1,7 @@
+config:
+  epc:
+    enable_pcap: true
+  enb:
+    enable_pcap: true
+  modem:
+    enable_pcap: true