bug#21174: 24.5; please make message-do-fcc respect local settings of message-handler-fcc-function

[David Bremner]

I'm trying to wrap message-mail in a function that (effectively)
temporarily sets the value of message-fcc-handler-function. This isn't
as easy as let-binding the variable since message-do-fcc is invoked
after the call to message-mail returns. The obvious next thing to try is
setting the variable buffer-locally. This fails because message-do-fcc
creates a new buffer and copies the buffer content from the message-mode
buffer, but not any of the buffer-local variables.

In GNU Emacs 24.5.1 (x86_64-pc-linux-gnu, X toolkit, Xaw3d scroll bars)
 of 2015-06-28 on trouble, modified by Debian
Windowing system distributor `The X.Org Foundation', version 11.0.11702000
System Description:     Debian GNU/Linux testing (stretch)

Configured using:
 `configure --build x86_64-linux-gnu --prefix=/usr
 --sharedstatedir=/var/lib --libexecdir=/usr/lib
 --localstatedir=/var/lib --infodir=/usr/share/info
 --mandir=/usr/share/man --with-pop=yes
 
--enable-locallisppath=/etc/emacs24:/etc/emacs:/usr/local/share/emacs/24.5/site-lisp:/usr/local/share/emacs/site-lisp:/usr/share/emacs/24.5/site-lisp:/usr/share/emacs/site-lisp
 --build x86_64-linux-gnu --prefix=/usr --sharedstatedir=/var/lib
 --libexecdir=/usr/lib --localstatedir=/var/lib
 --infodir=/usr/share/info --mandir=/usr/share/man --with-pop=yes
 
--enable-locallisppath=/etc/emacs24:/etc/emacs:/usr/local/share/emacs/24.5/site-lisp:/usr/local/share/emacs/site-lisp:/usr/share/emacs/24.5/site-lisp:/usr/share/emacs/site-lisp
 --with-x=yes --with-x-toolkit=lucid --with-toolkit-scroll-bars
 --without-gconf --without-gsettings 'CFLAGS=-g -O2
 -fstack-protector-strong -Wformat -Werror=format-security -Wall'
 CPPFLAGS=-D_FORTIFY_SOURCE=2 LDFLAGS=-Wl,-z,relro'