POSIX prohibits any library function from setting errno to 0.

Correct my previous commit and add a comment to the manpage
indicating that the user must set errno to 0 if they wish to
distinguish "no such user" from "error".

Pointed out by: Jacques Vidrine (nectar@)

lib/libc/gen/getpwent.3

@@ -185,9 +185,13 @@ return a valid pointer to a passwd structure on success
 or
 .Dv NULL
 if the entry is not found or if an error occurs.
-In the latter case,
+If an error does occur,
 .Va errno
 will be set.
+Note that programs must explicitly set
+.Va errno
+to zero before calling any of these functions if they need to
+distinguish between a non-existent entry and an error.
 The functions
 .Fn getpwent_r ,
 .Fn getpwnam_r ,

lib/libc/gen/getpwent.c

@@ -388,7 +388,7 @@ getpw(int (*fn)(union key, struct passwd *, char *, size_t, struct passwd **),
 				return (NULL);
 		}
 	} while (res == NULL && rv == ERANGE);
-	if (res == NULL)
+	if (rv != 0)
 		errno = rv;
 	return (res);
 }