Haiku support for Nim (#8542)

* posix_other: Haiku now has spawn.h

This is added per https://dev.haiku-os.org/ticket/13446

* posix_other: Add Haiku specific Dirent members

* cpuinfo: Add an implementation for Haiku

* distros: Add basic Haiku support

* encodings: update Haiku support

* fenv, math: Haiku now provides libm

* times: Add Haiku struct members

* ansi_c, osalloc: Add Haiku constants

* threads: Add Haiku support

* testament: Haiku uses LIBRARY_PATH

* nim.cfg: Update Haiku support

libnetwork should only be linked if network functions are used

* threads: Haiku does not support -pthread switch

* tworkingdir: Haiku's env is in /bin

* posix_other: add SIGKILLTHR for Haiku

* sockets: link with libnetwork on Haiku

* coro: correct ucontext.h location

http://pubs.opengroup.org/onlinepubs/009696699/basedefs/ucontext.h.html

* coro: ucontext backend is not available on Haiku

Haiku doesn't provide the <ucontext.h> header, as it was removed from POSIX

* coro: fix setjmp backend

The compiler does not allow statements after a noreturn function

* nativesockets: Haiku doesn't support AI_V4MAPPED

* system: hostOS can contains "haiku"

* os: add support for Haiku's packagefs

packagefs is read-only, but there are writable holes to the underlying
file system as well

* os: update constant for Haiku

tests/testament/categories.nim

@@ -109,10 +109,14 @@ proc runBasicDLLTest(c, r: var TResults, cat: Category, options: string) =
     safeCopyFile("lib" / nimrtlDll, "tests/dll" / nimrtlDll)
   else:
     # posix relies on crappy LD_LIBRARY_PATH (ugh!):
-    var libpath = getEnv"LD_LIBRARY_PATH".string
+    const libpathenv = when defined(haiku):
+                         "LIBRARY_PATH"
+                       else:
+                         "LD_LIBRARY_PATH"
+    var libpath = getEnv(libpathenv).string
     # Temporarily add the lib directory to LD_LIBRARY_PATH:
-    putEnv("LD_LIBRARY_PATH", "tests/dll" & (if libpath.len > 0: ":" & libpath else: ""))
-    defer: putEnv("LD_LIBRARY_PATH", libpath)
+    putEnv(libpathenv, "tests/dll" & (if libpath.len > 0: ":" & libpath else: ""))
+    defer: putEnv(libpathenv, libpath)
     var nimrtlDll = DynlibFormat % "nimrtl"
     safeCopyFile("lib" / nimrtlDll, "tests/dll" / nimrtlDll)