kconfig: replace current_pos with separate cur_{filename,lineno}

Replace current_pos with separate variables representing the file name
and the line number, respectively.

No functional change is intended.

By the way, you might wonder why the "<none>" fallback exists in
zconf_curname(). menu_add_symbol() saves the current file and the line
number. It is intended to be called only during the yyparse() time.
However, menu_finalize() calls it, where there is no file being parsed.
This is a long-standing hack that should be fixed later. I left a FIXME
comment.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>