Find one occurrence of ABBREV, and return the expansion.
DIRECTION > 0 means look that many times backwards.
DIRECTION < 0 means look that many times forward.
DIRECTION = 0 means try both backward and forward.
IGNORE-CASE non-nil means ignore case when searching.
This sets `dabbrev--last-direction' to 1 or -1 according
to the direction in which the occurrence was actually found.
It sets `dabbrev--last-expansion-location' to the location
of the start of the occurrence.
(fn ABBREV DIRECTION IGNORE-CASE)