Add ability to turn a cpulimit-ed process's limit up or down on the fly

[DrHyde]

Send SIGUSR1 or SIGUSR2 to cpulimit to turn the limit up or down by 1 %age point at a time.

[HiGarfield]

The access to limit is not atomic.

I have an improved solution with atomic access.

https://github.com/HiGarfield/cpulimit/commit/cdbdb3744314788f839f516f3de3f34bb7bc9f58

To be compatible with old compilers, my implementation is based on the c89atomic library.