bin/47361: /bin/sh incorrectly applies IFS to $barwhen expanding "${foo-$bar}"

[dsl%netbsd.org@localhost]

>Number:         47361
>Category:       bin
>Synopsis:       /bin/sh incorrectly applies IFS to $barwhen expanding 
>"${foo-$bar}"
>Confidential:   no
>Severity:       serious
>Priority:       medium
>Responsible:    bin-bug-people
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Sat Dec 22 19:55:00 +0000 2012
>Originator:     David Laight
>Release:        All
>Organization:
NetBSD
>Environment:
Any /bin/sh with expand.h 1.87 or earlier (or equivalent for release branches)
>Description:
/bin/sh incorrectly applies filed splitting when expanding the alternate
text for ${foo-$bar} and ${foo+$bar}.

For Greg Troxel on tech-userlevel.
>How-To-Repeat:
sh -c 'a="x  x"; printf "%s\n" "${PWD+$a}"'
should output "x  x" (without the quotes) on a single line,
buggy versions output one x one each of two lines.
>Fix: