ucm: use closefrom instead of close_range

closefrom is a library function with a fallback mechanism for when the
kernel does not support the close_range syscall.

Also check for the function properly instead of assuming it is available
with _GNU_SOURCE defined.

Closes: #486
Fixes: #485
Signed-off-by: Mike Gilbert <floppym@gentoo.org>
Signed-off-by: Jaroslav Kysela <perex@perex.cz>

Author: floppym

configure.ac

@@ -52,6 +52,7 @@ dnl Checks for library functions.
 AC_PROG_GCC_TRADITIONAL
 AC_CHECK_FUNCS([uselocale])
 AC_CHECK_FUNCS([eaccess])
+AC_CHECK_DECLS([closefrom])
 
 dnl Enable largefile support
 AC_SYS_LARGEFILE

src/ucm/ucm_exec.c

@@ -259,8 +259,8 @@ int uc_mgr_exec(const char *prog)
 
 		close(f);
 
-#if defined(_GNU_SOURCE)
-		close_range(3, maxfd, 0);
+#if HAVE_DECL_CLOSEFROM
+		closefrom(3);
 #else
 		for (f = 3; f < maxfd; f++)
 			close(f);