scmutil: make termwidth() obtain stdio from ui

I'm getting rid of direct sys.stderr|out|in references so Py3 porting will
be slightly easier.

mercurial/scmposix.py

@@ -41,11 +41,11 b' def userrcpath():'
     else:
         return [os.path.expanduser('~/.hgrc')]
 
-def termwidth():
+def termwidth(ui):
     try:
         import array
         import termios
-        for dev in (sys.stderr, sys.stdout, sys.stdin):
+        for dev in (ui.ferr, ui.fout, ui.fin):
             try:
                 try:
                     fd = dev.fileno()

mercurial/scmwindows.py

@@ -53,5 +53,5 b' def userrcpath():'
         path.append(os.path.join(userprofile, '.hgrc'))
     return path
 
-def termwidth():
+def termwidth(ui):
     return win32.termwidth()

mercurial/ui.py

@@ -822,7 +822,7 b' class ui(object):'
                 return int(encoding.environ['COLUMNS'])
             except ValueError:
                 pass
-        return scmutil.termwidth()
+        return scmutil.termwidth(self)
 
     def formatted(self):
         '''should formatted output be used?