permit read -N n (n ≠ -1) and read -t to return partial reads with $? == 1

issue spotted by carstenh, could have been a documentation ambiguity issue
(as -N was designed to read and return exactly n bytes), but this resolves
it in a way both backwards-compatible and user-pleasing

funcs.c

@@ -38,7 +38,7 @@
 #endif
 #endif
 
-__RCSID("$MirOS: src/bin/mksh/funcs.c,v 1.272 2015/05/01 23:16:29 tg Exp $");
+__RCSID("$MirOS: src/bin/mksh/funcs.c,v 1.273 2015/06/28 14:57:24 tg Exp $");
 
 #if HAVE_KILLPG
 /*
@@ -1964,8 +1964,9 @@ c_read(const char **wp)
 			break;
 		case 0:
 			/* timeout expired for this call */
-			rv = 1;
-			goto c_read_out;
+			bytesread = 0;
+			/* fake EOF read; all cases return 1 */
+			goto c_read_didread;
 		default:
 			bi_errorf("%s: %s", Tselect, cstrerror(errno));
 			rv = 2;
@@ -1990,6 +1991,7 @@ c_read(const char **wp)
 		goto c_read_readloop;
 	}
 
+ c_read_didread:
 	switch (readmode) {
 	case READALL:
 		if (bytesread == 0) {
@@ -2012,7 +2014,7 @@ c_read(const char **wp)
 		if (bytesread == 0) {
 			/* end of file reached */
 			rv = 1;
-			xp = Xstring(xs, xp);
+			/* may be partial read: $? = 1, but content */
 			goto c_read_readdone;
 		}
 		xp += bytesread;

mksh.1

@@ -1,4 +1,4 @@
-.\" $MirOS: src/bin/mksh/mksh.1,v 1.367 2015/05/23 17:43:20 tg Exp $
+.\" $MirOS: src/bin/mksh/mksh.1,v 1.368 2015/06/28 14:57:25 tg Exp $
 .\" $OpenBSD: ksh.1,v 1.159 2015/03/25 12:10:52 jca Exp $
 .\"-
 .\" Copyright © 2002, 2003, 2004, 2005, 2006, 2007, 2008, 2009,
@@ -74,7 +74,7 @@
 .\" with -mandoc, it might implement .Mx itself, but we want to
 .\" use our own definition. And .Dd must come *first*, always.
 .\"
-.Dd $Mdocdate: May 23 2015 $
+.Dd $Mdocdate: June 28 2015 $
 .\"
 .\" Check which macro package we use, and do other -mdoc setup.
 .\"
@@ -3832,7 +3832,8 @@ if empty, instead of the ASCII newline character as input line delimiter.
 .It Fl N Ar z
 Instead of reading till end-of-line, read exactly
 .Ar z
-bytes; less if EOF or a timeout occurs.
+bytes.
+If EOF or a timeout occurs, a partial read is returned with exit status 1.
 .It Fl n Ar z
 Instead of reading till end-of-line, read up to
 .Ar z
@@ -3851,6 +3852,9 @@ The argument must immediately follow the option character.
 Interrupt reading after
 .Ar n
 seconds (specified as positive decimal value with an optional fractional part).
+The exit status of
+.Nm read
+is 1 if the timeout occurred, but partial reads may still be returned.
 .It Fl r
 Normally, the ASCII backslash character escapes the special
 meaning of the following character and is stripped from the input;