Posix from detect (#5697)

* refactor posix.nim
* types move to separate files for platform-specifc and generic
("other')
* consts move to separate files that get autogenerated by detect.nim
* proc's stay where they are for now, though in a second stage might
move as well
* fix missing when

tools/detect/.gitignore

@@ -0,0 +1,5 @@
+pre.c
+pre.i
+genconsts.c
+other_consts.nim
+linux_amd64_consts.nim