Make skeleton a part of DFA.

Make skeleton a part of DFA.

This let us create skeletom right after DFA creation (but befor DFA
has been mangled in different ways), but call skeleton methods any time.

Undefined control flow is now checked at the time of real code generation,
that's why all those tests that use '-r' changed: re2c stopped reporting
'rules:re2c' blocks and reports 'use:re2c' blocks instead.