I'm sick of all those #ifdefs in settings.c, and in any case plink

I'm sick of all those #ifdefs in settings.c, and in any case plink
and pterm need at least one default setting to be _different_ (pterm
needs the default term type to be `xterm', while plink needs it to
be taken from $TERM). So here's a completely new alternative
mechanism for platform- and app-specific default settings. Ben will
probably want to check the integrity of the Mac port, since I've
fiddled with it without testing that it still compiles.

git-svn-id: svn://svn.tartarus.org/sgt/putty@2513 cda61777-01e9-0310-a592-d414129be87e