manually touch atime when reading a mbox file

The only common use of atime left is local mail agents leaving a mark on
mbox file after the mail has been read.  And, since POSIX-2008, it is
possible to use futimens() to alter atime even on filesystems mounted with
noatime.

There's no extra cost for doing this on when atime updates are enabled:
the inode will be dirty already.

configure.ac

@@ -448,6 +448,9 @@ AC_CHECK_FUNCS(ftruncate, , [AC_CHECK_LIB(x, chsize)])
 dnl SCO has strftime() in libintl
 AC_CHECK_FUNCS(strftime, , [AC_CHECK_LIB(intl, strftime)])
 
+dnl Set the atime of files
+AC_CHECK_FUNCS(futimens)
+
 dnl AIX may not have fchdir()
 AC_CHECK_FUNCS(fchdir, , [mutt_cv_fchdir=no])
 

main.c

@@ -392,6 +392,12 @@ static void show_version (void)
 #else
 	"-HAVE_RESIZETERM  "
 #endif
+	
+#ifdef HAVE_FUTIMENS
+	"+HAVE_FUTIMENS  "
+#else
+	"-HAVE_FUTIMENS  "
+#endif
         );	
   
   puts (

mbox.c

@@ -436,6 +436,7 @@ static int mbox_open_mailbox (CONTEXT *ctx)
     rc = mmdf_parse_mailbox (ctx);
   else
     rc = -1;
+  mutt_touch_atime (fileno (ctx->fp));
 
   mbox_unlock_mailbox (ctx);
   mutt_unblock_signals ();
@@ -1257,6 +1258,8 @@ int mutt_reopen_mailbox (CONTEXT *ctx, int *index_hint)
     return (-1);
   }
 
+  mutt_touch_atime (fileno (ctx->fp));
+
   /* now try to recover the old flags */
 
   index_hint_set = (index_hint == NULL);

muttlib.c

@@ -1942,6 +1942,16 @@ void mutt_set_mtime (const char* from, const char* to)
   }
 }
 
+/* set atime to current time, just as read() would do on !noatime.
+ * Silently ignored if unsupported. */
+void mutt_touch_atime (int f)
+{
+#ifdef HAVE_FUTIMENS
+  struct timespec times[2]={{0,UTIME_NOW},{0,UTIME_OMIT}};
+  futimens(f, times);
+#endif
+}
+
 const char *mutt_make_version (void)
 {
   static char vstring[STRING];

protos.h

@@ -122,6 +122,7 @@ time_t mutt_local_tz (time_t);
 time_t mutt_mktime (struct tm *, int);
 time_t mutt_parse_date (const char *, HEADER *);
 int is_from (const char *, char *, size_t, time_t *);
+void mutt_touch_atime (int);
 
 const char *mutt_attach_fmt (
 	char *dest,