MFC r202324: In 'make delete-old', use 'exec' to redirect an fd persistently

MFC r202324: In 'make delete-old', use 'exec' to redirect an fd persistently

That is, write 'exec 3<&0' instead of '3<&0'. Due to an sh(1) bug fixed in
head in r199953, the latter also persisted, provided that fd 3 was not open
before.  With newer sh or fd 3 open, it would not delete orphaned catpages.

git-svn-id: svn://svn.freebsd.org/base/stable/8@203133 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f