Cygwin: ps: fix compiler warning in ttynam

Cygwin: ps: fix compiler warning in ttynam

The helper function ttynam creates a tty name by using sprintf wrongly
on a pretty short buffer.  The foramt string only specifies a minimum
field length, not a maximum field length, so gcc-9.2.0 complains:

  ps.cc:101:23: warning: 'sprintf' may write a terminating nul past the
  end of the destination [-Wformat-overflow=]

Fix this thoroughly by specifying a maximum field width as well as by
using snprintf with a fixed buffer length.  Also, drop using a static
buffer in favor of using a buffer in the caller.

Signed-off-by: Corinna Vinschen <corinna@vinschen.de>