[perl #114040] Parse formats in interpolating constructs

[perl #114040] Parse formats in interpolating constructs

For re-evals, this is something that broke recently, post-5.16 (the
jumbo fix).  For other interpolating constructs, this has never
worked, as far as I can tell.

The lexer was losing track of PL_lex_state (aka PL_parser->lex_state)
when parsing formats.  Usually, the state alternates between
LEX_FORMLINE (a picture line) and LEX_NORMAL (an argument line), but
the LEX_NORMAL should actually be whatever the state was before the
format started.

This commit adds a new parser member to track the ‘normal’ state when
parsing a format.

It also tweaks S_scan_formline to handle multi-line buffers outside of
string eval (such as happens in interpolating constructs).

That bufend assignment that is removed as a result is not necessary as
of a0d0e21ea6ea (perl 5.000).  That very commit added a bufend assign-
ment after the sv_gets (later filter_gets; later lex_next_chunk) fur-
ther down in the loop in scan_formline.