dot/Xdefaults, Makefile: Replace `putty-defaults' with X resources.

dot/Xdefaults, Makefile: Replace `putty-defaults' with X resources.

This is more cumbersome to set up, but ends up being more useful.  Using
X resources, it's possible to have a number of different profiles for
`pterm' which can be switched using the `-name' option.  Also, this
allows for overriding on the command-line using `-xrm', which doesn't
work if the settings are defined in `pterm''s own configuration file.