sh: Allow unquoted newlines in word in ${param+word} etc.

POSIX requires accepting unquoted newlines in word in parameter expansions
like ${param+word}, ${param#word}, although the Bourne shell did not support
it, it is not commonly used and might make it harder to find a missing
closing brace.

It was also strange that something like

foo="${bar#
}"

was rejected.

Reported by: Martijn Dekker via Robert Elz