scanner: introduce start condition stack

scanner: introduce start condition stack

Add a small initial chunk of flex start conditionals.

This starts with two low-hanging fruits, numgen and j/symhash.

NUMGEN and HASH start conditions are entered from flex when
the corresponding expression token is encountered.

Flex returns to the INIT condition when the bison parser
has seen a complete numgen/hash statement.

This intentionally uses a stack rather than BEGIN()
to eventually support nested states.

The scanner_pop_start_cond() function argument is not used yet, but
will need to be used later to deal with nesting.

Signed-off-by: Florian Westphal <fw@strlen.de>