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

bin/sh/parser.c

@@ -1434,7 +1434,8 @@ readtoken1(int firstc, char const *initialsyntax, const char *eofmark,
 
 			switch(synentry) {
 			case CNL:	/* '\n' */
-				if (state[level].syntax == BASESYNTAX)
+				if (level == 0 &&
+				    state[level].syntax == BASESYNTAX)
 					goto endword;	/* exit outer loop */
 				USTPUTC(c, out);
 				plinno++;

bin/sh/tests/expansion/Makefile

@@ -84,6 +84,7 @@ ${PACKAGE}FILES+=	plus-minus5.0
 ${PACKAGE}FILES+=	plus-minus6.0
 ${PACKAGE}FILES+=	plus-minus7.0
 ${PACKAGE}FILES+=	plus-minus8.0
+${PACKAGE}FILES+=	plus-minus9.0
 ${PACKAGE}FILES+=	question1.0
 ${PACKAGE}FILES+=	readonly1.0
 ${PACKAGE}FILES+=	redir1.0
@@ -101,5 +102,7 @@ ${PACKAGE}FILES+=	trim6.0
 ${PACKAGE}FILES+=	trim7.0
 ${PACKAGE}FILES+=	trim8.0
 ${PACKAGE}FILES+=	trim9.0
+${PACKAGE}FILES+=	trim10.0
+${PACKAGE}FILES+=	trim11.0
 
 .include <bsd.test.mk>

bin/sh/tests/expansion/plus-minus9.0

@@ -0,0 +1,8 @@
+# $FreeBSD$
+
+a=1
+b=${a+
+}
+n='
+'
+[ "$b" = "$n" ]

bin/sh/tests/expansion/trim10.0

@@ -0,0 +1,7 @@
+# $FreeBSD$
+
+a='z
+'
+b=${a%
+}
+[ "$b" = z ]

bin/sh/tests/expansion/trim11.0

@@ -0,0 +1,7 @@
+# $FreeBSD$
+
+a='z
+'
+b="${a%
+}"
+[ "$b" = z ]