Isolation: Rename NXT_HAVE_CLONE -> NXT_HAVE_LINUX_NS.

Due to the need to replace our use of clone/__NR_clone on Linux with
fork(2)/unshare(2) for enabling Linux namespaces(7) to keep the
pthreads(7) API working.  Let's rename NXT_HAVE_CLONE to
NXT_HAVE_LINUX_NS, i.e name it after the feature, not how it's
implemented, then in future if we change how we do namespaces again we
don't have to rename this.

Reviewed-by: Alejandro Colomar <alx@nginx.com>
Signed-off-by: Andrew Clayton <a.clayton@nginx.com>

auto/isolation

@@ -4,7 +4,7 @@
 # Linux clone syscall.
 
 NXT_ISOLATION=NO
-NXT_HAVE_CLONE=NO
+NXT_HAVE_LINUX_NS=NO
 NXT_HAVE_CLONE_NEWUSER=NO
 NXT_HAVE_MOUNT=NO
 NXT_HAVE_UNMOUNT=NO
@@ -12,21 +12,21 @@ NXT_HAVE_ROOTFS=NO
 
 nsflags="USER NS PID NET UTS CGROUP"
 
-nxt_feature="clone(2)"
-nxt_feature_name=NXT_HAVE_CLONE
+nxt_feature="Linux unshare()"
+nxt_feature_name=NXT_HAVE_LINUX_NS
 nxt_feature_run=no
 nxt_feature_incs=
 nxt_feature_libs=
-nxt_feature_test="#include <sys/wait.h>
-                  #include <sys/syscall.h>
+nxt_feature_test="#define _GNU_SOURCE
+                  #include <sched.h>
 
                   int main(void) {
-                      return SYS_clone | SIGCHLD;
+                      return unshare(0);
                   }"
 . auto/feature
 
 if [ $nxt_found = yes ]; then
-    NXT_HAVE_CLONE=YES
+    NXT_HAVE_LINUX_NS=YES
 
     # Test all isolation flags
     for flag in $nsflags; do

auto/sources

@@ -299,7 +299,7 @@ if [ "$NXT_HAVE_HPUX_SENDFILE" = "YES" \
 fi
 
 
-if [ "$NXT_HAVE_CLONE" = "YES" ]; then
+if [ "$NXT_HAVE_LINUX_NS" = "YES" ]; then
     NXT_LIB_SRCS="$NXT_LIB_SRCS $NXT_LIB_CLONE_SRCS"
 fi