Refactor zfs_context.h to build on FreeBSD

- on Linux move Linux specific headers to zfs_context_os.h
- on FreeBSD move FreeBSD specific definitions to zfs_context_os.h
- remove duplicate tsd_ definitions
- remove unused AT_TYPE

Reviewed-by: Jorgen Lundman <lundman@lundman.net>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Don Brady <don.brady@delphix.com>
Signed-off-by: Matt Macy <mmacy@FreeBSD.org>
Closes #9668

lib/libspl/include/os/linux/sys/Makefile.am

@@ -7,4 +7,5 @@ libspl_HEADERS = \
 	$(top_srcdir)/lib/libspl/include/os/linux/sys/param.h \
 	$(top_srcdir)/lib/libspl/include/os/linux/sys/stat.h \
 	$(top_srcdir)/lib/libspl/include/os/linux/sys/sysmacros.h \
-	$(top_srcdir)/lib/libspl/include/os/linux/sys/uio.h
+	$(top_srcdir)/lib/libspl/include/os/linux/sys/uio.h \
+	$(top_srcdir)/lib/libspl/include/os/linux/sys/zfs_context_os.h

lib/libspl/include/os/linux/sys/zfs_context_os.h

@@ -19,12 +19,7 @@
  *
  * CDDL HEADER END
  */
-/*
- * Copyright 2006 Sun Microsystems, Inc.  All rights reserved.
- * Use is subject to license terms.
- */
-
-#ifndef _LIBSPL_SYS_BITMAP_H
-#define	_LIBSPL_SYS_BITMAP_H
 
+#ifndef ZFS_CONTEXT_OS_H
+#define	ZFS_CONTEXT_OS_H
 #endif

lib/libspl/include/sys/Makefile.am

@@ -4,7 +4,6 @@ libspldir = $(includedir)/libspl/sys
 libspl_HEADERS = \
 	$(top_srcdir)/lib/libspl/include/sys/acl.h \
 	$(top_srcdir)/lib/libspl/include/sys/acl_impl.h \
-	$(top_srcdir)/lib/libspl/include/sys/bitmap.h \
 	$(top_srcdir)/lib/libspl/include/sys/callb.h \
 	$(top_srcdir)/lib/libspl/include/sys/cmn_err.h \
 	$(top_srcdir)/lib/libspl/include/sys/cred.h \