sh: Do IFS splitting on word in ${v+word} and ${v-word}.

The code is inspired by NetBSD sh somewhat, but different because we
preserve the old Almquist/Bourne/Korn ability to have an unquoted part in a
quoted ${v+word}. For example, "${v-"*"}" expands to $v as a single field if
v is set, but generates filenames otherwise.

Note that this is the only place where we split text literally from the
script (the similar ${v=word} assigns to v and then expands $v). The parser
must now add additional markers to allow the expansion code to know whether
arbitrary characters in substitutions are quoted.

Example:
  for i in ${$+a b c}; do echo $i; done

Exp-run done by:	pav (with some other sh(1) changes)

1 files changed, 2 insertions, 0 deletions

diff --git a/bin/sh/expand.h b/bin/sh/expand.h
index a3e079a..a4bb198 100644
--- a/bin/sh/expand.h
+++ b/bin/sh/expand.h
@@ -52,6 +52,8 @@ struct arglist {
 #define	EXP_VARTILDE	0x4	/* expand tildes in an assignment */
 #define	EXP_REDIR	0x8	/* file glob for a redirection (1 match only) */
 #define EXP_CASE	0x10	/* keeps quotes around for CASE pattern */
+#define EXP_SPLIT_LIT	0x20	/* IFS split literal text ${v+-a b c} */
+#define EXP_LIT_QUOTED	0x40	/* for EXP_SPLIT_LIT, start off quoted */
 
 
 union node;