set proper initial value for pcap snaplen

This pull requests fixes a infinite loop within libpcap.
One condition for this loop is the initial value of `GBL_PCAP->snaplen` set to **INT32_MAX** which means **0x7fffffff**.

The loop occurs in *pcap-linux.c* in the function `create_ring()`:
```c
	req.tp_block_size = getpagesize();
	while (req.tp_block_size < req.tp_frame_size)
		req.tp_block_size <<= 1;
```

If the *ethtool* can not be leveraged to read the offload capability and the MTU, the value of *req.tp_frame_size* is derived from `GBL_PCAP->snaplen`, otherwize it's derived from the interface MTU which doesn't introduce a problem.

Some more offset is beeing added to the snaplen so that the value of *req.tp_frame_size* is on my system **0x80000050** (note that *req.tp_frame_size* is of type *unsigned int*).
The value of *req.tp_block_size* (as well *unsigned int*) is on my system **0x00001000**.

Now the while loop bit-shifts *req.tp_block_size* until it's greater than *req.tp_frame_size*. Unfortunately under these initial conditions, this is not possible. The iterations of the loop look like:
```
initial tp_block_size: 0x00001000, tp_frame_size: 0x80000050
tp_block_size: 0x00001000 < tp_frame_size: 0x80000050 - shifting tp_block_size: 0x00002000
tp_block_size: 0x00002000 < tp_frame_size: 0x80000050 - shifting tp_block_size: 0x00004000
tp_block_size: 0x00004000 < tp_frame_size: 0x80000050 - shifting tp_block_size: 0x00008000
tp_block_size: 0x00008000 < tp_frame_size: 0x80000050 - shifting tp_block_size: 0x00010000
tp_block_size: 0x00010000 < tp_frame_size: 0x80000050 - shifting tp_block_size: 0x00020000
tp_block_size: 0x00020000 < tp_frame_size: 0x80000050 - shifting tp_block_size: 0x00040000
tp_block_size: 0x00040000 < tp_frame_size: 0x80000050 - shifting tp_block_size: 0x00080000
tp_block_size: 0x00080000 < tp_frame_size: 0x80000050 - shifting tp_block_size: 0x00100000
tp_block_size: 0x00100000 < tp_frame_size: 0x80000050 - shifting tp_block_size: 0x00200000
tp_block_size: 0x00200000 < tp_frame_size: 0x80000050 - shifting tp_block_size: 0x00400000
tp_block_size: 0x00400000 < tp_frame_size: 0x80000050 - shifting tp_block_size: 0x00800000
tp_block_size: 0x00800000 < tp_frame_size: 0x80000050 - shifting tp_block_size: 0x01000000
tp_block_size: 0x01000000 < tp_frame_size: 0x80000050 - shifting tp_block_size: 0x02000000
tp_block_size: 0x02000000 < tp_frame_size: 0x80000050 - shifting tp_block_size: 0x04000000
tp_block_size: 0x04000000 < tp_frame_size: 0x80000050 - shifting tp_block_size: 0x08000000
tp_block_size: 0x08000000 < tp_frame_size: 0x80000050 - shifting tp_block_size: 0x10000000
tp_block_size: 0x10000000 < tp_frame_size: 0x80000050 - shifting tp_block_size: 0x20000000
tp_block_size: 0x20000000 < tp_frame_size: 0x80000050 - shifting tp_block_size: 0x40000000
tp_block_size: 0x40000000 < tp_frame_size: 0x80000050 - shifting tp_block_size: 0x80000000
tp_block_size: 0x80000000 < tp_frame_size: 0x80000050 - shifting tp_block_size: 0x00000000
tp_block_size: 0x00000000 < tp_frame_size: 0x80000050 - shifting tp_block_size: 0x00000000
[...]
```

This can be easily reproduced by running ... 
```
ettercap -Tqilo
```
My wifi interface is also not capable to be queried by **ethtool**, so using this interface, the issue occurs as well. My Ethernet interface has no issues, as the MTU is being used rather than the initial value of `GBL_PCAP->snaplen`.

Resetting the value to the default of 65535 (**UINT16_MAX**) fixes the issue.