mount_smb: lots of improvements

1. Revamped how autossh is killed.  It was previously hanging around
   after the script exited.  More /etc/sudoers hacks are needed for
   this.  There's a new script, `removepidfile' that is used to kill
   autossh.  It needs to run via sudo, so it's (hopefully) locked down
   and is secure.

2. Changed the way the main body of the script executes.  Put
   everything inside:
      {
        # code here
	exit
      }
   which allows me to modify the script while it's running.  YAY!!
   This is a nice hack.

3. Make the script handle a bad SMB password.  That part was broken at
   some point.