Introduce the ability to control whether the shell run in pterm is a

Introduce the ability to control whether the shell run in pterm is a
login shell or not. Also moved these new pieces of configuration
into the Config structure, though they won't stay there forever
since they will need to be moved out into platform-dependent config.

git-svn-id: http://svn.tartarus.org/sgt/putty@2060 cda61777-01e9-0310-a592-d414129be87e