dehacked: Fix BEX string expansions.

The Dehacked code must pass a special flag to DEH_ReadLine() when
parsing the [STRINGS] section, so that lines can be escaped onto
following lines. However, this flag value was calculated incorrectly.

The BEX [STRINGS] section cannot be looked up at the start of
parsing a Dehacked file, because GetSectionByName() will return
NULL for it (extended strings are not available yet). Instead,
simplify the logic to look at the name of the current section and
check that way.