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>

src/nxt_process.h

@@ -7,13 +7,13 @@
 #ifndef _NXT_PROCESS_H_INCLUDED_
 #define _NXT_PROCESS_H_INCLUDED_
 
-#if (NXT_HAVE_CLONE)
+#if (NXT_HAVE_LINUX_NS)
 #include <unistd.h>
 #include <nxt_clone.h>
 #endif
 
 
-#if (NXT_HAVE_CLONE)
+#if (NXT_HAVE_LINUX_NS)
 /*
  * Old glibc wrapper for getpid(2) returns a cached pid invalidated only by
  * fork(2) calls.  As we use clone(2) for container, it returns the wrong pid.
@@ -100,7 +100,7 @@ typedef struct {
     nxt_cgroup_t             cgroup;
 #endif
 
-#if (NXT_HAVE_CLONE)
+#if (NXT_HAVE_LINUX_NS)
     nxt_clone_t              clone;
 #endif