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

lib/pure/distros.nim

@@ -126,6 +126,8 @@ type
     OpenBSD
     DragonFlyBSD
 
+    Haiku
+
 
 const
   LacksDevPackages* = {Distribution.Gentoo, Distribution.Slackware,
@@ -166,6 +168,8 @@ proc detectOsImpl(d: Distribution): bool =
   of Distribution.Solaris:
     let uname = toLowerAscii(uname())
     result = ("sun" in uname) or ("solaris" in uname)
+  of Distribution.Haiku:
+    result = defined(haiku)
   else:
     let dd = toLowerAscii($d)
     result = dd in toLowerAscii(uname()) or dd in toLowerAscii(release())
@@ -224,6 +228,8 @@ proc foreignDepInstallCmd*(foreignPackageName: string): (string, bool) =
       result = ("pacman -S " & p, true)
     else:
       result = ("<your package manager here> install " & p, true)
+  elif defined(haiku):
+    result = ("pkgman install " & p, true)
   else:
     result = ("brew install " & p, false)