tlibthread: fix nbrecvul, recvul to match man page, Plan 9 - plan9port - [fork] Plan 9 from user space
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       ---
   DIR commit 369923f6fca5a713698dbac76c486ddb28f43721
   DIR parent 93f9789c048d9bfd75192adce3ed9a3ff8a68fbc
  HTML Author: Russ Cox <rsc@swtch.com>
       Date:   Mon, 13 Jan 2020 17:00:27 -0500
       
       libthread: fix nbrecvul, recvul to match man page, Plan 9
       
       They return 0 on failure, not -1.
       Bug introduced in my original libthread-for-Unix code.
       
       Fixes #230.
       
       Diffstat:
         M src/libthread/channel.c             |       4 ++--
       
       1 file changed, 2 insertions(+), 2 deletions(-)
       ---
   DIR diff --git a/src/libthread/channel.c b/src/libthread/channel.c
       t@@ -397,7 +397,7 @@ chanrecvul(Channel *c)
        
                if(_chanop(c, CHANRCV, &val, 1) > 0)
                        return val;
       -        return -1;
       +        return 0;
        }
        
        int
       t@@ -413,5 +413,5 @@ channbrecvul(Channel *c)
        
                if(_chanop(c, CHANRCV, &val, 0) > 0)
                        return val;
       -        return -1;
       +        return 0;
        }