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/osproc/texitsignal.nim

@@ -28,9 +28,12 @@ if paramCount() == 0:
     # windows kill happens using TerminateProcess(h, 0), so we should get a
     # 0 here
     echo p.waitForExit() == 0
+  elif defined(haiku):
+    # on Haiku, the program main thread receive SIGKILLTHR
+    echo p.waitForExit() == 128 + SIGKILLTHR
   else:
     # on posix (non-windows), kill sends SIGKILL
     echo p.waitForExit() == 128 + SIGKILL
 
 else:
-  sleep(5000)  # should get killed before this
+  sleep(5000)  # should get killed before this

tests/osproc/tworkingdir.nim

@@ -12,6 +12,8 @@ else:
   var process: Process
   when defined(android):
     process = startProcess("/system/bin/env", "/system/bin", ["true"])
+  elif defined(haiku):
+    process = startProcess("/bin/env", "/bin", ["true"])
   else:
     process = startProcess("/usr/bin/env", "/usr/bin", ["true"])
   let dir2 = getCurrentDir()