Rejecting non-Linux pivot_root(2).

Some non-Linux systems implement pivot_root(2), even if they
don't document that.  An example is MacOS:

$ grepc pivot_root / 2>/dev/null
.../sys/sysproto.h:3012:
int pivot_root(struct proc *, struct pivot_root_args *, int *);

Since the prototype of the syscall differs from that of Linux, we
can't use that syscall.  Let's make sure the test only detects
pivot_root(2) under Linux.  Also, rename the feature macro to make
clear that it's only about Linux's pivot_root(2).

This closes #737 issue on GitHub.

auto/isolation

@@ -61,11 +61,14 @@ fi
 
 
 nxt_feature="Linux pivot_root()"
-nxt_feature_name=NXT_HAVE_PIVOT_ROOT
+nxt_feature_name=NXT_HAVE_LINUX_PIVOT_ROOT
 nxt_feature_run=no
 nxt_feature_incs=
 nxt_feature_libs=
 nxt_feature_test="#include <sys/syscall.h>
+                  #if !defined(__linux__)
+                  # error
+                  #endif
 
                   int main() {
                       return SYS_pivot_root;