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/system.nim

@@ -1365,7 +1365,7 @@ const
   hostOS* {.magic: "HostOS".}: string = ""
     ## a string that describes the host operating system. Possible values:
     ## "windows", "macosx", "linux", "netbsd", "freebsd", "openbsd", "solaris",
-    ## "aix", "standalone".
+    ## "aix", "haiku", "standalone".
 
   hostCPU* {.magic: "HostCPU".}: string = ""
     ## a string that describes the host CPU. Possible values: