The relation between parser_param#base_block and #in_main were very
subtle.
A main script (that is passed via a command line) was parsed under
base_block = TOPLEVEL_BINDING and in_main = 1.
A script loaded by Kernel#require was parsed under
base_block = NULL and in_main = 0.
If base_block is non-NULL and in_main == 0, it is parsed by Kernel#eval
or family.

However, we know that TOPLEVEL_BINDING has no local variables when a
main script is parsed.  So, we don't have to parse a main script under
base_block = TOPLEVEL_BINDING.

Instead, this change parses a main script under base_block = 0.
If base_block is non-NULL, it is parsed by Kernel#eval or family.
By this simplication, "in_main" is no longer needed.