sh: Allow terminating a heredoc with a terminator at EOF without a newline.

This is sometimes used with eval or old-style command substitution, and most
shells other than ash derivatives allow it.

It can also be used with scripts that violate POSIX's requirement on the
application that they end in a newline (scripts must be text files except
that line length is unlimited).

Example:
v=`cat <<EOF
foo
EOF`
echo $v

This commit does not add support for the similar construct with new-style
command substitution, like
v=$(cat <<EOF
foo
EOF)
This continues to require a newline after the terminator.

bin/sh/parser.c

@@ -1513,10 +1513,12 @@ checkend: {
 
 				p = line;
 				for (q = eofmark + 1 ; *q && *p == *q ; p++, q++);
-				if (*p == '\n' && *q == '\0') {
+				if ((*p == '\0' || *p == '\n') && *q == '\0') {
 					c = PEOF;
-					plinno++;
-					needprompt = doprompt;
+					if (*p == '\n') {
+						plinno++;
+						needprompt = doprompt;
+					}
 				} else {
 					pushstring(line, strlen(line), NULL);
 				}

tools/regression/bin/sh/parser/heredoc11.0

@@ -0,0 +1,26 @@
+# $FreeBSD$
+
+failures=''
+
+check() {
+	if eval "[ $* ]"; then
+		:
+	else
+		echo "Failed: $*"
+		failures=x$failures
+	fi
+}
+
+check '`cat <<EOF
+foo
+EOF` = foo'
+
+check '"`cat <<EOF
+foo
+EOF`" = foo'
+
+check '`eval "cat <<EOF
+foo
+EOF"` = foo'
+
+test "x$failures" = x