bug#20727: 24.5; Font fallback doesn't work for the Emoji range

[Paul Eggert <eggert@cs.ucla.edu>]

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The recent change to fontsel.el messed up my display when I was trying 
to test obsolescent 8-bit environments.  I can reproduce the problem on 
Fedora 21 x86-64 by running:

src/emacs -Q -font 
-misc-fixed-medium-r-semicondensed--0-0-75-75-c-0-iso8859-1

and then by typing:

abc C-x 8 [ def C-x 8 ] ghi

This inserts "abc‘def’ghi" into *scratch*.  The display looks like "abc  
‘def’  ghi" with huge spaces around the quotes (see attached 
screenshot).  There are similar problems with many other characters.  
Apparently this is because I don't have the Symbola font installed, so 
the change caused Emacs to fall back on Chinese double-width quotes 
rather than on the font it used before (which is -misc-liberation 
serif-medium-r-normal--13-94-100-100-p-74-iso10646-1 for the above 
invocation of Emacs).

I imagine that there will be similar problems with choosing Symbola even 
if it's available, as users may prefer the font they've selected 
explicitly, so perhaps the change should be enabled only for characters 
that are not already available in the user-selected font?  In the 
meantime, in master commit 203e84c6cf9b8356e376cc748b5ed331df96dc9e I 
worked around my problem by skipping the change if Symbola is not installed.

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