Add a NetBSD PAL.

Currently untested, but identical to the FreeBSD one so should work...
The NetBSD man pages for `madvise` and `mmap` mention the flags that we
use.

src/pal/pal.h

@@ -13,6 +13,7 @@ namespace snmalloc
 #  include "pal_free_bsd_kernel.h"
 #  include "pal_freebsd.h"
 #  include "pal_linux.h"
+#  include "pal_netbsd.h"
 #  include "pal_openbsd.h"
 #  include "pal_windows.h"
 #endif
@@ -33,6 +34,8 @@ namespace snmalloc
     PALFreeBSDKernel;
 #  elif defined(__FreeBSD__)
     PALFBSD;
+#  elif defined(__NetBSD__)
+    PALNetBSD;
 #  elif defined(__OpenBSD__)
     PALOBSD;
 #  else