libbe: fix build against sysutils/openzfs, part 1

This is the half of the changes required that work as-is with both in-tree
ZFS and the new hotness, sysutils/openzfs.  Highlights are less dependency
on header pollution (from somewhere) and using 'mnttab' instead of
'extmnttab'.   In the in-tree ZFS, the latter is a #define for the former,
but in the port extmnttab is actually a distinct struct that's a super-set
of mnttab.  We really want mnttab here anyways, so just use it.

lib/libbe/be.c

@@ -34,6 +34,9 @@ __FBSDID("$FreeBSD$");
 #include <sys/stat.h>
 #include <sys/ucred.h>
 
+#include <sys/zfs_context.h>
+#include <sys/mntent.h>
+
 #include <ctype.h>
 #include <libgen.h>
 #include <libzfs_core.h>
@@ -67,7 +70,7 @@ static int
 be_locate_rootfs(libbe_handle_t *lbh)
 {
 	struct statfs sfs;
-	struct extmnttab entry;
+	struct mnttab entry;
 	zfs_handle_t *zfs;
 
 	/*

lib/libbe/be_access.c

@@ -31,6 +31,8 @@
 #include <sys/cdefs.h>
 __FBSDID("$FreeBSD$");
 
+#include <sys/mntent.h>
+
 #include "be.h"
 #include "be_impl.h"
 

lib/libbe/be_info.c

@@ -30,6 +30,8 @@
 #include <sys/cdefs.h>
 __FBSDID("$FreeBSD$");
 
+#include <sys/zfs_context.h>
+
 #include "be.h"
 #include "be_impl.h"