Issue2 eval cost posix timer rebase next 2.27

Rebased branch, updated indentation and FLUFF_POSIX_TIMERS_H naming.  Left thread timer alone (see comment on annotation), left int type alone (trying to maintain consistency with the uvalarm implementation and compatibility 32-bit builds; I presently do not have a 32-bit VM set up for testing, and consider changing the existing eval.c/uvalarm.c code to be out-of-scope for this issue.)