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/Makefile.am

@@ -27,7 +27,6 @@ USER_C = \
 	strlcat.c \
 	strlcpy.c \
 	timestamp.c \
-	zone.c \
 	include/sys/list.h \
 	include/sys/list_impl.h
 
@@ -35,7 +34,8 @@ if BUILD_LINUX
 USER_C += \
 	os/linux/getexecname.c \
 	os/linux/gethostid.c \
-	os/linux/getmntany.c
+	os/linux/getmntany.c \
+	os/linux/zone.c
 endif
 
 if BUILD_FREEBSD
@@ -43,7 +43,8 @@ USER_C += \
 	os/freebsd/getexecname.c \
 	os/freebsd/gethostid.c \
 	os/freebsd/getmntany.c \
-	os/freebsd/mnttab.c
+	os/freebsd/mnttab.c \
+	os/freebsd/zone.c
 endif
 
 libspl_la_SOURCES = \