bug#56496: 28.1; add-abbrev should respect active region

[Howard Melman]

add-mode-abbrev and add-global-abbrev use the text in the
buffer as the expansion and then prompt for an abbrev.  If
given a numeric argument it's used as the number of words to
use as the expansion or if 0 to use the region.

I think it would be nice if these commands respected an
active region and just used it without having to use a 0 arg.

-- 

Howard