log(): don't abort if we fail to write to stderr.

Failing to write to the log sucks, but not as much as failing to clean up
just because stderr disappeared.  So let's catch any IOError exception from
log() and just ignore it.

This should fix a problem reported by Camille Moncelier, which is that
sshuttle firewall entries stick around if your tty dies strangely (eg. your
X server aborts for some reason).

helpers.py

@@ -4,9 +4,14 @@ logprefix = ''
 verbose = 0
 
 def log(s):
-    sys.stdout.flush()
-    sys.stderr.write(logprefix + s)
-    sys.stderr.flush()
+    try:
+        sys.stdout.flush()
+        sys.stderr.write(logprefix + s)
+        sys.stderr.flush()
+    except IOError:
+        # this could happen if stderr gets forcibly disconnected, eg. because
+        # our tty closes.  That sucks, but it's no reason to abort the program.
+        pass
 
 def debug1(s):
     if verbose >= 1: