rework help messages for fee-related options

- mention the units and show the default for -paytxfee
small cleanup of #ifdefs in BindListenPort()

- SO_NOSIGPIPE isn't available on WIN32 so merge the 2 non-WIN32 blocks
- use predefined names from header for IPV6_PROTECTION_LEVEL and
  PROTECTION_LEVEL_UNRESTRICTED
add missing BOOST_FOREACH indentation in ThreadSocketHandler()
RPC client: Simplify command line string-to-JSON-value conversion code

By default, all command line parameters are converted into JSON string
values.  There is no need to manually specify the incoming type.

A binary decision "parse as string or JSON?" is all that's necessary.

Convert to a simple class, initialized at runtime startup, which offers
a quick lookup to answer "parse as JSON?" conversion question.

Future parameter conversions need only to indicate the method name
and zero-based index of the parameter needing JSON parsing.
CBloomFilter::clear() method
Relay double-spends, subject to anti-DOS

Allows network wallets and other clients to see transactions that respend
a prevout already spent in an unconfirmed transaction in this node's mempool.

Knowledge of an attempted double-spend is of interest to recipients of the
first spend.  In some cases, it will allow these recipients to withhold
goods or services upon being alerted of a double-spend that deprives them
of payment.

As before, respends are not added to the mempool.

Anti-Denial-of-Service-Attack provisions:
 - Use a bloom filter to relay only one respend per mempool prevout
 - Rate-limit respend relays to a default of 100 thousand bytes/minute
 - Define tx2.IsEquivalentTo(tx1): equality when scriptSigs are not considered
 - Do not relay these equivalent transactions

Remove an unused variable declaration in txmempool.cpp.
UI to alert of respend attempt affecting wallet.

Respend transactions that conflict with transactions already in the
wallet are added to it.  They are not displayed unless they also involve
the wallet, or get into a block.  If they do not involve the wallet,
they continue not to affect balance.

Transactions that involve the wallet, and have conflicting non-equivalent
transactions, are highlighted in red.  When the conflict first occurs, a
modal dialog is thrown.

CWallet::SyncMetaData is changed to sync only to equivalent transactions.

When a conflict is added to the wallet, counter nConflictsReceived is
incremented.  This acts like a change in active block height for the
purpose of triggering UI updates.
Add -respendnotify option and new RPC data

-respendnotify=<cmd> Execute command when a network tx respends wallet
tx input (%s=respend TxID, %t=wallet TxID)

Add respendsobserved array to gettransaction, listtransactions, and
listsinceblock RPCs.  This omits the malleated clones that are included
in the walletconflicts array.

Add RPC help for respendsobserved and walletconflicts (help was missing
for the latter).
Add release notes entry
Formatting, spelling, comment fixes.
Move non-trivial uint256.h methods to uint256.cpp
fix RPC error replies

After pull #4288, RPC messages indicating errors have a Content-Length unrelated
to their actual contents, rendering bitcoin-cli and curl unable to decode the
reply.

This patch sets the Content-Length field based on the actual content returned.

Additionally, pull #4288 clobbered the error descriptions provided in
ErrorReply, which bitcoin-cli relies upon; this patch moves #4288 http-error
descriptions to an HTTPError method, allowing HTTPReply to pass content on
unchanged.
Track peers' available blocks
Add a skiplist to the CBlockIndex structure.

This allows fast (O(log n)) access to far predecessor blocks.
Use it to speed up CChain::FindFork and CChain::GetLocator.
Add skiplist unit tests
Merge pull request #4440

16f33f1 fix RPC error replies (kazcw)
Merge pull request #4437

de79aaa Move non-trivial uint256.h methods to uint256.cpp (Pieter Wuille)
Merge pull request #4420 from sipa/skiplist

Add a skiplist to the CBlockIndex structure.
Merge pull request #3883 from dgenr8/first_double_spend

Relay and alert user to double spends
Fix the build for Qt5

Merging #3883 broke the Qt5 build, define the color in the standard
way.