bug#10077: 23.3; Rmail: rmail-summary-by-regexp doesn't work because of missing rmail-search-mime-header-function

[Mark Lillibridge <mark.lillibridge@hp.com>]

Reproducing is easy: go to an rmail-buffer and hit M-C-s:

    C-M-s (translated from <escape> C-s) runs the command
    rmail-summary-by-regexp, which is an interactive compiled Lisp
    function in `rmailsum.el'.
    
    It is bound to C-M-s, <menu-bar> <summary> <regexp>.
    
    (rmail-summary-by-regexp REGEXP)
    
    Display a summary of all messages according to regexp REGEXP.
    If the regular expression is found in the header of the message
    (including in the date and other lines, as well as the subject line),
    Emacs will list the message in the summary.

You'll get the following error:

rmail-message-regexp-p-1: You must set `rmail-search-mime-header-function'


- Mark