Search for something that could be used to expand ABBREV.
Second arg, REVERSE, is t for reverse search, nil for forward.
The variable `dabbrev-limit' controls the maximum search region size.
Third argument IGNORE-CASE non-nil means treat case as insignificant while
looking for a match and when comparing with previous matches. Also if
that's non-nil and the match is found at the beginning of a sentence
and is in lower case except for the initial then it is converted to
all lower case for return.
Table of expansions already seen is examined in buffer
`dabbrev--last-table' so that only distinct possibilities are found
Returns the expansion found, or nil if not found.
Leaves point at the location of the start of the expansion. (fn ABBREV REVERSE IGNORE-CASE)