Major destabilisation, phase 1. In this phase I've moved (I think)

Major destabilisation, phase 1. In this phase I've moved (I think)
all the global and function-static variables out of terminal.c into
a dynamically allocated data structure. Note that this does not yet
confer the ability to run more than one of them in the same process,
because other things (the line discipline, the back end) are still
global, and also in particular the address of the dynamically
allocated terminal-data structure is held in a global variable
`term'. But what I've got here represents a reasonable stopping
point at which to check things in. In _theory_ this should all still
work happily, on both Unix and Windows. In practice, who knows?

[originally from svn r2115]