Another ugly mutable global variable gone: now, instead of this
variable being defined in cmdline.c and written to by everyone's
main(), it's defined _alongside_ everyone's main() as a constant, and
cmdline.c just refers to it.

A bonus is that now nocmdline.c doesn't have to define it anyway for
tools that don't use cmdline.c. But mostly, it didn't need to be
mutable, so better for it not to be.

While I'm at it, I've also fiddled with the bit flags that go in it,
to define their values automatically using a list macro instead of
manually specifying each one to be a different power of 2.