When a -nofoo option is seen, instead of adding it to a separate
set of negated args, set the arg as being an empty vector of strings.

This changes the behaviour in some ways:

• -nofoo=0 still sets foo=1 but no longer treats it as a negated arg
• -nofoo=1 -foo=2 has GetArgs() return [2] rather than [2,0]
• "foo=2 \n -nofoo=1" in a config file no longer returns [2,0], just [0]
• GetArgs returns an empty vector for negated args

Partial backport of core PR11862 (commit 4d34fcc)

Depends on D2432