* tests/calc.at: Exercise prologue splitting.

* data/bison.simple, data/bison.c++: Use `b4_pre_prologue' and
`b4_post_prologue' instead of `b4_prologue'.
* src/output.c (prepare): Add the `pre_prologue' and `post_prologue'
muscles.
(output): Free pre_prologue_obstack and post_prologue_obstack.
* src/files.h, src/files.c (attrs_obstack): Remove.
(pre_prologue_obstack, post_prologue_obstack): New.
* src/reader.c (copy_definition): Add a parameter to specify the
obstack to fill, instead of using attrs_obstack unconditionally.
(read_declarations): Pass pre_prologue_obstack to copy_definition if
`%union' has not yet been seen, pass post_prologue_obstack otherwise.