Gnus problem with nil font in face definition (was: Default mm-uu-extract face hard to read on ttys)

[Ted Zlatanov <tzz@lifelogs.com>]

With the latest Emacs from CVS and the latest Gnus from CVS, many Gnus
faces look strange.  They were fine with checkouts from about a month
ago.

It seems that the cause is faces that have the font set to nil, instead
of unspecified.  I don't know if this is a Gnus-only problem, and it may
be caused by an Emacs problem, so I am cc-ing both emacs-devel and ding.

For example, describe-face on message-header-to says:

Face: message-header-to (sample) (customize this face)
Documentation: Face used for displaying From headers.
Defined in `message.el'.

        Family: unspecified
         Width: unspecified
        Height: unspecified
        Weight: bold
         Slant: unspecified
    Foreground: DarkOliveGreen1
    Background: unspecified
     Underline: unspecified
      Overline: unspecified
Strike-through: unspecified
           Box: unspecified
       Inverse: unspecified
       Stipple: unspecified
          Font: nil
       Fontset: unspecified
       Inherit: unspecified

but message-header-name is:

Face: message-header-name (sample) (customize this face)
Documentation: Face used for displaying header names.
Defined in `message.el'.

        Family: unspecified
         Width: unspecified
        Height: unspecified
        Weight: unspecified
         Slant: unspecified
    Foreground: green
    Background: unspecified
     Underline: unspecified
      Overline: unspecified
Strike-through: unspecified
           Box: unspecified
       Inverse: unspecified
       Stipple: unspecified
          Font: unspecified
       Fontset: unspecified
       Inherit: unspecified

For me, message-header-name appears normal, using the right font from
my X defaults:

#define FONT_ISO_XL -misc-fixed-medium-r-normal--20-200-75-75-c-100-iso10646-1
Emacs*font: FONT_ISO_XL

but message-header-to is set in a large proportional font, which looks
ugly.  describe-font crashed my Emacs session, so I'm not sure about the
specifics of the font that was used.

Ted