Add zpool_nextboot, move zfs_jail to libzfs.h

FreeBSD has a zfsbootcfg command that wants zpool_nextboot in libzfs.

Add the function to FreeBSD's libzfs_compat.c, and while here move
the prototype for zfs_jail out of param.h in FreeBSD's SPL and into
libzfs.h under an ifdef for FreeBSD, where the prototype for
zpool_nextboot joins it.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Ryan Moeller <ryan@iXsystems.com>
Closes #10524

lib/libspl/include/os/freebsd/sys/param.h

@@ -61,10 +61,4 @@ extern size_t spl_pagesize(void);
 
 extern int execvpe(const char *name, char * const argv[], char * const envp[]);
 
-struct zfs_handle;
-/*
- * Attach/detach the given filesystem to/from the given jail.
- */
-extern int zfs_jail(struct zfs_handle *zhp, int jailid, int attach);
-
 #endif