We are computing our file descriptors limits based on whether we use
poll or select. However, we are taking that into account only partially
(subtracting from fd_max in one case, but from nFD later on). Moreover,
nBind is also only accounted for partially.

Simplify and fix this logic

The current logic for file descriptor accounting is pretty convoluted and hard
to follow. This is partially caused by the lack of documentation plus non-intuitive
variable naming (which was more intuitive when fewer things were accounted for, but
hasn't aged well). This has led to this accounting being error-prone and hard to maintain
(as shown in he previous commit).

Redefine some of the constants, plus document what are we accounting for so this can be
extended more easily

Remove FreeBSD workaround to bitcoin#2695