Add printing pcap stats; improve logs formatting

Epcap prints pcap statistics for received and dropped packets
in verbose mode. The statistics can be printed at a certain interval.
Existing logs has better formatting now.
epcap C program accepts a new option for setting the interval: -S
followed by an integer and epcap:start/1 accepts
an {stat_interval, integer()} option.
(Motivated by FlowForwarding/LINC-Switch#283).
Allow setting pcap internal buffer size

The buffer size can be adjusted by passing -b option with an integer
value to epcap C program. Erlang epcap:start/1 function accepts an
{buffer_size, integer()} option.
(Motivated by FlowForwarding/LINC-Switch#283).