PACKET_MMAP version 1/2 improvements

### Description

The fdbased LinkEndpoint supports PACKET_MMAP, but is missing a few useful improvements:

- TX: we TX using `sendmmsg`, but should use PACKET_MMAP buffers.
- RX: the PACKET_MMAP dispatcher seems to return 1-3 packets at a time, while the recvmmsg dispatcher often returns 8. We should investigate this, as PACKET_MMAP _should_ return many packets at once and ultimately be faster than recvmmsg.

**This is specific to PACKET_MMAP TPACKET_V1 and TPACKET_V2, not V3.** V3 is unsuitable as a dispatcher because it markedly increases latency (by O(milliseconds)) in order to reduce CPU usage.

### Is this feature related to a specific bug?

No

### Do you have a specific solution in mind?

I believe we only have 32 slots (`tpFrameNR`) in the dispatcher. Could that be limiting the number of returned packets?