Introduce <sys/qmath.h>, a fixed-point math library from Netflix.

This makes it possible to perform mathematical operations
on

fractional values without using floating point. It operates on Q

numbers, which are integer-sized, opaque structures
initialized

to hold a chosen number of integer and fractional
bits.

For a general description of the Q number system, see the "Fixed Point

Representation & Fractional Math" whitepaper[1]; for the actual

API see the qmath(3) man page.

This is one of dependencies for the upcoming stats(3)

framework[2] that will be applied to the TCP stack in

a later commit.

- https://www.superkits.net/whitepapers/Fixed%20Point%20Representation%20&%20Fractional%20Math.pdf
- https://reviews.freebsd.org/D20477

Sponsored By: Klara Inc, Netflix

Obtained from: Netflix