libspl: Provide platform-specific zone implementations

FreeBSD has the concept of jails, a precursor to Solaris's zones, which
can be mapped to the required zones interface with relative ease.  The
previous ZFS implementation in FreeBSD did so, and we should continue
to provide an appropriate implementation in OpenZFS as well.

Move lib/libspl/zone.c into platform code and adopt the correct
implementation for FreeBSD.

While here, prune unused code.

Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Ryan Moeller <freqlabs@FreeBSD.org>
Closes #10851

lib/libspl/include/zone.h

@@ -26,25 +26,16 @@
 #ifndef _LIBSPL_ZONE_H
 #define	_LIBSPL_ZONE_H
 
-
-
 #include <sys/types.h>
 #include <sys/zone.h>
-#include <sys/priv.h>
 
 #ifdef	__cplusplus
 extern "C" {
 #endif
 
 #define	GLOBAL_ZONEID		0
-#define	GLOBAL_ZONEID_NAME	"global"
 
-/*
- * Functions for mapping between id and name for active zones.
- */
 extern zoneid_t		getzoneid(void);
-extern zoneid_t		getzoneidbyname(const char *);
-extern ssize_t		getzonenamebyid(zoneid_t, char *, size_t);
 
 #ifdef	__cplusplus
 }