net/tap: add Rx trigger

This commit adds a signal-based trigger to the Rx burst function in order
to avoid unnecessary system calls while Rx queues are empty.

Triggered Rx bursts put less pressure on the kernel, free up CPU resources
for applications and result in a noticeable performance improvement when
sharing CPU threads with other PMDs.

Measuring the traffic forwarding rate between two physical devices in
testpmd (IO mode, single thread, 64B packets) before and after adding two
tap PMD instances (4 ports total) that do not process any traffic and
comparing results yields:

Without Rx trigger:

 -15% (--burst=32)
 -62% (--burst=1)

With Rx trigger:

 -0.3% (--burst=32)
 -6% (--burst=1)

Signed-off-by: Adrien Mazarguil <adrien.mazarguil@6wind.com>
Acked-by: Pascal Mazon <pascal.mazon@6wind.com>
Acked-by: Keith Wiles <keith.wiles@intel.com>