Split options into constant and mutable; restricted access to mutable ones.

Split options into constant and mutable; restricted access to mutable ones.

Some options are immutable (target, output files; global switches like
conditions, reuse mode, storable states; support of flex syntax, etc.).
These options are passed as command-line arguments and never change.
It is safe to read them from any program point after parsing command-line
arguments.

Other options are configurable; they have block scope (may be specified
anywhere inside of the block and still affect the whole block).
Reading mutable options of yet unparsed block is not allowed because
they may affect the way RE2C parses current block (RE2C would be tempted
to base decisions on the latest option value, which may not be the final
one).