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

…ult section

When specified in bitcoin.conf without using the [regtest] or [test]
section header, or a "regtest." or "test." prefix, the "addnode",
"connect", "port", "bind", "rpcport", "rpcbind", and "wallet" settings
will only be applied when running on mainnet.

When network-specific options such as -addnode, -connect, etc are
specified in the default section of the config file, but that setting is
ignored due to testnet or regtest being in use, and it is not overridden
by either a command line option or a setting in the [regtest] or [test]
section of the config file, a warning is added to the log, eg:

  Warning: Config setting for -connect only applied on regtest network when in [regtest] section.