dot/*: Major shake-up of shell configuration.

dot/*: Major shake-up of shell configuration.

Split out the major logic of session startup and shell configuration
into shell-neutral files `dot/profile' and `dot/shell-rc'.  There's a
little support now for pleasant `dash' sessions, but since `dash'
doesn't have a line editor or any way of varying the prompt dynamically,
it's still not marvellous.

Various things which have also been changed:

  * The prompt setup has been heavily refactored.  There's now a piece
    which calculates most of the major pieces, and another piece spliced
    into the shell's command-loop machinery to insert stuff like the Git
    status and previous exit code.  This also takes care of setting and
    clearing the Screen status, so that's much cleaner now.  (In
    particular, this means that Screen doesn't say that it's running
    Bash when it isn't really.)

  * Many of the shell functions have been revised to improve
    portability.  The old hack behind Solaris man(1) has gone
    because (a) it's hard to write portably, and (b) I don't use Solaris
    any more anyway.