cleanup: use flex-generated declarations for scanner interface

We now pass the --header=... option to flex.  This causes it to
generate a header file describing the flex interface.  We #include
this almost everywhere we #include conffile_internal.h, and that makes
the declarations of yyin and yylex in conffile_internal.h redundant so
remove them.

Note that flex generates a conffile.yy.c which also contains many of
the same declarations as in conffile.yy.h.  So, unfortunately, we
should not #include conffile.yy.h in conffile.hh.c and therefore not
in conffile_internal.h either (if we want -Wredundant-decls, which we do).

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>