units: close stderr of subshell to suppress error messages

When ctags exits with abnormal status, its parent shell
reports it to stderr. This is noise for our purpose. This
patch makes the subshell close its stderr.

This bug(noise) is reported by in #60 by @vhda.
This patch reverts wrong fix (59cfb8df658162b5ba852c7bddbbd4a8d0b14f10)
for the same bug.

Signed-off-by: Masatake YAMATO <yamato@redhat.com>