c++: Fix up -fcontract* options

I've noticed
+FAIL: compiler driver --help=c++ option(s): "^ +-.*[^:.]\$" absent from output: "  -fcontract-build-level=[off|default|audit] Specify max contract level to generate runtime checks for"
error, this is due to missing dot at the end of the description.

The second part of the first hunk should fix that, but while at it,
I find it weird that some options don't have RejectNegative, yet
for options that accept an argument a negative option looks weird
and isn't really handled.

Though, shall we have those [on|off] options at all?
Those are inconsistent with all other boolean options gcc has.
Every other boolean option is -fwhatever for it being on
and -fno-whatever for it being off, shouldn't the options be
without arguments and accept negatives (-fcontract-assumption-mode
vs. -fno-contract-assumption-mode etc.)?

2022-11-23  Jakub Jelinek  <jakub@redhat.com>

	* c.opt (fcontract-assumption-mode=, fcontract-continuation-mode=,
	fcontract-role=, fcontract-semantic=): Add RejectNegative.
	(fcontract-build-level=): Terminate description with dot.