improvements to the [STRINGS] section parser

- restructure DEH_ReadLine() to avoid use of "goto"
- bex_string_t type name ends in "_t"
- declare constant table as "static const"
- add magic comment *allow-extended-strings* and corresponding
  variable deh_allow_extended_strings
- fix logical error when no [STRINGS] section is registered to which
  corrent_section could be compared