Replace Chia/XCH References with Silicoin/SIT

[WarutaShinken]

One of my complaints about Silicoin is that it uses the CHIA_ROOT environment variable to determine the location of the mainnet directory. This can cause Silicoin to overwrite Chia's files, such as wallets, configs, certs and worst of all, the 40 GB blockchain database if not accounted for.

In this PR, I have replaced all instances of CHIA_ROOT with SIT_ROOT, so that users may define a custom mainnet directory location for Silicoin and Chia without them clashing with eachother. The same has been done with CHIA_KEYS_ROOT => SIT_KEYS_ROOT.

EDIT: Since the writing of this PR, I have added the following changes:

• Changed the UPNP name from chia to sit.
• Replaced various occurrences of xch with sit.
  • This includes a functional change: The config will now read the farmer and pool target addresses from sit_target_address instead of xch_target_address.

[WarutaShinken]

closed due to being superseeded by https://github.com/zcomputerwiz/silicoin-blockchain/tree/waruta-patches