`tail +1c` doesn't work when stdin is from terminal

Running in xterm:
```
falsifian moth ~ $ 9 tail +1c
hello
hello
falsifian moth ~ $ echo hello | 9 tail +1c
ello
```
For some reason, in the first case, it's not skipping the "h". The effect is similar with numbers other than 1.

This is `plan9port-20210323` from OpenBSD ports, which should be at commit  88a87fadae6629932d9c160f53ad5d79775f8f94