daemon: Do not use 'vfork'.

This fixes random EBADF errors in the daemon on ancient Linux kernels
such as version 2.6.32.

* config-daemon.ac: Remove detection of 'vfork'.

config-daemon.ac

@@ -78,10 +78,13 @@ if test "x$guix_build_daemon" = "xyes"; then
   dnl lutimes and lchown: used when canonicalizing store items.
   dnl posix_fallocate: used when extracting archives.
   dnl vfork: to speed up spawning of helper programs.
+  dnl   `--> now disabled because of unpredictable behavior:
+  dnl        see <http://lists.gnu.org/archive/html/guix-devel/2014-05/msg00036.html>
+  dnl        and Nix commit f794465c (Nov. 2012).
   dnl sched_setaffinity: to improve RPC locality.
   dnl statvfs: to detect disk-full conditions.
   dnl strsignal: for error reporting.
-  AC_CHECK_FUNCS([lutimes lchown posix_fallocate vfork sched_setaffinity \
+  AC_CHECK_FUNCS([lutimes lchown posix_fallocate sched_setaffinity \
      statvfs nanosleep strsignal])
 
   dnl Check whether the store optimiser can optimise symlinks.