Fix generation through pcap file

Fix generation through pcap file

- When using "pcap file" argument in PROX config file in the gen mode
(and no submode), packets were truncated to 8 bytes (and a warning printed)
This has been fixed.
- "Set value" command was checking whether offset was within first 1518 bytes
unrespective to the mtu (e.g. jumbo packets). This has been fixed.
- There was a limit of 64K packets loaded from a pcap file (which could be
overwritten through PROX config). This limit has been removed - by default
all packets from pcap are loaded
- When pcap was loaded, memory was allocated to  store the packet. However
just enough memory was allocated, so that there was no way to change (increase)
packet size through command line after that. Now memory is allocated to store
up to mtu size
- While "set value" command was acting on all packets, "pkt size" command
was only changing the first packet. This has now been changed and both commands
change all the packets.
- A few other minor cleanups.

Change-Id: I7264cee8d57e3e6a405dfd7e7a52a95b99399d99
Signed-off-by: Xavier Simonart <xavier.simonart@intel.com>