DOCS: Update developers.txt with hints for POSIX/WIN32 portable coding

Over time several patterns emerged about things that behave differently in WIN32 vs. usual "POSIX" builds of NUT.

Some of these typically require `ifdef` code-path branching and different variable types involved (signals, etc.); while others are already generalized (checking for valid/invalid file descriptors).

Gotta make this knowledge less arcane!