[9fans] Common code - isatty

[Einar Karttunen <ekarttun@cs.helsinki.fi>]

Hello

While reading source code I noticed several different isatty
implementations. They are slightly different and I would be interested
in 1) which is the correct implementation and 2) whether a patch
to fix the others would be welcome and whether a common location 
in a library or cut-paste is preferred.

The locations include at least:
ape/lib/ap/isatty.c      - check whether fd2path ends in "/dev/cons"
cmd/ip/telnet.c          - dirstat -> "cons"
cmd/rc/plan9.c           - dirstat -> "ptty" or same dev,type,qid as "/dev/cons"
cmd/ssh/ssh.c            - fd2path ends in "/dev/cons"
cmd/upas/common/libsys.c - dirstat -> "cons"
libstdio/setvbuf.c       - dirstat -> same dev,type and qid as "/dev/cons"

- Einar Karttunen