Make pflua-quickcheck into a tool which can test multiple properties.

Refactored pflua-quickcheck to generate pflua IR and test properties
independently.
Removed second return value from choose

It was never being used, and caused invalid IR to be generated when the second
return value was being treated as an extra argument to operators.
Updated the README to reflect current use
Test pflua math against libpcap math

For now, only test positive numbers, and math within the range where doubles
can exactly represent all integers.
This avoids triggering the following known issues:
"Pflua does not allow negative numbers; tcpdump does"
https://github.com/Igalia/pflua/issues/82
and
"Pflua: large multiplication uses floats, leading to different results"
https://github.com/Igalia/pflua/issues/83
Fixed bugs/nits dpino noticed

- lines are now <= 80 characters
- 'local' declarations added/removed as necessary
- stale util import removed
- Used %x to match hex
- ("a string"):format() syntax now used
- Removed commented-out code
- Reformatted odd if/else line
- Fixed prop_opt_eq_unopt bugs from wrongly declaring local
- Fixed prop_opt_eq_unopt bugs from partly-applied renames
- Normalized lua file permissions to 664
- Clearer/better output on test failure
- Added a trivial fail property
- Reduced default math check iterations to 50 (calling tcpdump is slow)
Updated to newest pflua

This avoids having the math property trigger a fixed division bug.
Travis-ci and Makefile tcpdump-awareness

* Added a script to check for tcpdump -d being usable.
* Made .travis.yml install tcpdump and use setcap.
* Added a tcpdumpchecks target.
* Conditionally run the tcpdumpchecks target if tcpdump -d is usable.
Localized 4 variables
Merged with master
Merge branch 'refactor_and_add_mathcheck'

This merge turns pflua-quickcheck into a tool that handles arbitrary (equal)
properties. It also adds several properties, including for testing pflua
math against tcpdump math.