Fix occasionally failing Solaris setwinsize()

Although this is expected to always happen from the parent
process, it does intermittently occur on the child process:

https://teamcity-master.pexpect.org/viewLog.html?buildId=561&buildTypeId=Pexpect_SunosBuild&tab=buildLog#_focus=1292

A similar issue is found in tmux, which is referenced inline.

( There are other errors found in this build log -- but I
  suspect it is due to the forked test runner. )
On my system, `errno.EXNIO` doesn't exist - did you mean ENXIO?