add parser for [STRINGS] sections in BEX files
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
simplify and generalize deh_allow_extended_strings usage
Merge pull request #435 from chocolate-doom/bexstr

Add support for the BEX extended string syntax.

Boom added an alternate method for overriding Dehacked strings, using the [STRINGS] section with special symbolic names for each of the strings that can possibly be replaced.

This format is used in the Freedoom DEHACKED lump (deliberately, so that the Freedoom WAD does not include the original text to be replaced). As we want to have support for the Freedoom IWADs with Chocolate Doom, add support for the BEX [STRINGS] section, but protected by a magic comment that must be included in Dehacked/BEX files that use it.