- reset file pointers to previous context after evaluating arguments
- in __fb_query_symbol__, __fb_eval__, __fb_arg_extract__, __fb_iif__,
- and after releasing lexer context
- add enum LEX_TKCTX_CONTEXT to specify the kind of lexer context
- LEX_TKCTX_CONTEXT_INIT    for initializing, first time only
- LEX_TKCTX_CONTEXT_INCLUDE for include file
- LEX_TKCTX_CONTEXT_EVAL    for preprocessor evaluation