posix: workaround lack of TIOCGWINSZ on Irix (issue3449)

On an Irix 6.5.24 system, TIOCGWINSZ is not available. This means that
any usage of the "hg" tool that looks up the terminal size (e.g. "hg
help") will fail with an AttributeError.

A simple work-around is just to wrap this block in mercurial/posix.py
with a try/except so that it ends up using the default 80 characters
width.

mercurial/posix.py

@@ -409,10 +409,13 b' def termwidth():'
                     continue
                 if not os.isatty(fd):
                     continue
-                arri = fcntl.ioctl(fd, termios.TIOCGWINSZ, '\0' * 8)
-                width = array.array('h', arri)[1]
-                if width > 0:
-                    return width
+                try:
+                    arri = fcntl.ioctl(fd, termios.TIOCGWINSZ, '\0' * 8)
+                    width = array.array('h', arri)[1]
+                    if width > 0:
+                        return width
+                except AttributeError:
+                    pass
             except ValueError:
                 pass
             except IOError, e: