Bring back close_range() support proper

As noted in 31baed29, my commit
0c1aef58 was wrong, and it was even
worse than we thought:

Despite what the linux man page suggests, the close_range()
declaration is in unistd.h on Linux like on freebsd.

We do not actually need linux/close_range.h, because it has only
macro definitions which we do not need.

We now add a specific configure test if close_range() not only exists
but also works.

Closes #3905

configure.ac

@@ -623,6 +623,27 @@ if test "$ac_cv_have_tcp_fastopen" = yes; then
 fi
 LIBS="${save_LIBS}"
 
+AC_CHECK_FUNCS([close_range])
+
+# Check for working close_range()
+if test "$ac_cv_func_close_range" = yes; then
+AC_CACHE_CHECK([if close_range is working],
+  [ac_cv_have_working_close_range],
+  [AC_RUN_IFELSE(
+    [AC_LANG_PROGRAM([[
+#include <unistd.h>
+    ]],[[
+return (close_range(0, 2, 0));
+    ]])],
+    [ac_cv_have_working_close_range=yes],
+    [ac_cv_have_working_close_range=no])
+  ])
+fi
+if test "x$ac_cv_have_working_close_range" = xyes; then
+   AC_DEFINE([HAVE_WORKING_CLOSE_RANGE], [1],
+	     [Define if OS has working close_range()])
+fi
+
 # Run-time directory
 if test "${localstatedir}" = '${prefix}/var' ; then
    VARNISH_STATE_DIR='/var/run'

lib/libvarnish/vsub.c

@@ -37,8 +37,10 @@
 #include <stdint.h>
 #include <stdlib.h>		// Solaris closefrom(3c)
 #include <string.h>
-#include <unistd.h>
-#ifndef HAVE_CLOSEFROM
+#include <unistd.h>		// BSD/Linux close_range(2)
+#ifdef HAVE_WORKING_CLOSE_RANGE
+#elif HAVE_CLOSEFROM
+#else
 #  include <dirent.h>
 #endif
 
@@ -65,7 +67,10 @@ VSUB_closefrom(int fd)
 
 	assert(fd >= 0);
 
-#ifdef HAVE_CLOSEFROM
+#ifdef HAVE_WORKING_CLOSE_RANGE
+	AZ(close_range(fd, ~0U, 0));
+	return;
+#elif HAVE_CLOSEFROM
 	closefrom(fd);
 	return;
 #else