sfxge: Separate software Tx queue limit for non-TCP traffic

[arybchik]

Summary

Add separate software Tx queue limit for non-TCP traffic to make total
limit higher and avoid local drops of TCP packets because of no
backpressure.
There is no point to make non-TCP limit high since without backpressure
UDP stream easily overflows any sensible limit.

Split early drops statistics since it is better to have separate counter
for each drop reason to make it unabmiguous.

Sponsored by: Solarflare Communications, Inc.

[gnn]

Remember to put Approved by: gnn (mentor) in the commit message.

[arybchik]

Committed to head