Enable entry searching functions to retrieve entry names longer than 8 characters

This adds a new longname field to lumpinfo_t, which contains the entry name without the path and extension, but without the 8-character limit from the WAD format.

This new field is used by W_CheckNumForName and W_CheckNumForNamePwad, enabling them to retrieve PK3 entries which names exceed 8 characters, which can be useful for Lua scripts that use custom HUD patches.

Also this renames lumpinfo_t.name2 to lumpinfo_t.fullname.