snc: generate header file

The top-level declaration parts of the generated code are now written to a
header file. These parts are (in the order listed):

- #includes of epicsTypes.h and seqCom.h
- definition of the _seq_var struct
- escaped code segments appearing before the first state set

The stdin/stdout redirection has been removed. Instead code generation now
uses the gen_code function exported from main. Switching between output
files is done using set_gen_h and set_gen_c, which are also exported from
main. This uses global (though static) variables inside main.c for
simplicity.