BUGFIX: Hashserver UI shows wrong server list when purchasing upgrades

[catloversg]

#1779 accidentally reverts the fix in #1665 and adds unexpected servers to the server list. How to reproduce this regression bug:

• Load a clean save file with SF9.
• Buy a hashserver.
• Buy a private server (p-server-0).
• Hacknet -> Spend Hashes on Upgrades -> Reduce Minimum Security -> Open the drop-down list. It will contain home, p-server-0, and hacknet-server-0.

The problem is in this code:

return s.hostname !== "home" && !purchased && !Player.hasAugmentation(AugmentationName.TheRedPill, true)
  ? s.hostname !== "w0r1d_d43m0n"
  : true;

When s is home, private servers, or hashservers, it always returns true.

While fixing this bug, I spotted other wrong logic in this file, so I fixed all of them in this PR. Those code paths are currently not used (ServerDropdown is only used by src\Hacknet\ui\HacknetUpgradeElem.tsx, and HacknetUpgradeElem.tsx only uses ServerType.Foreign.), so it's not a problem.

Code in isValidServer does not check members of ServerType properly:

• purchased is checked by (s instanceof Server && s.purchasedByPlayer) || s instanceof HacknetServer, but home also satisfies s instanceof Server && s.purchasedByPlayer, so purchased is in fact isOwnedServer (home, private servers, hashservers), not Everything from Owned except home computer.
• s.hostname !== "home" is used in wrong places.

@d0sboots To avoid making a breaking change with ns.hacknet.spendHashes (https://github.com/bitburner-official/bitburner-src/blob/d824cd4fa6692ca1e04173b892278b79587a88fb/src/Hacknet/HacknetHelpers.tsx), I don't touch it in this PR, but I want to hear your opinions about it:

• When it sees a hashserver, it returns false without printing a warning.
• Home and private servers are valid targets. It's inconsistent with how we determine the target in the UI. It's not exploitable, so it's not urgent.