sh +nounset and `for X; do` iteration fails if parameter set empty
by applying and testing FreeBSD's patch of Oct 24 2009 for this; see
http://svnweb.freebsd.org/base/head/bin/sh/expand.c?r1=198453&r2=198454
Also created an ATF test in tests/bin/sh/t_expand.sh for this error and
corrected a space->tabs problem there as well.