dot/mykermrc.in: Various tweaks:

  * Increase the command history size.

  * Build a more useful prompt string using unpleasant subterfuge.

  * Set the default file character set to UTF-8 independent of the
    locale -- which should determine the terminal character set but not
    files, which I decree ought to be UTF-8 regardless.  Emacs ought to
    sort this mess out for me.