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/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