Minor non-blocker updates to Crypto. #360

Inline comment and idea come from the encprivkeys branch
by Matt Corallo <matt@bluematt.me>.

This commit adds support for ckeys, or enCrypted private keys, to the wallet.
All keys are stored in memory in their encrypted form and thus the passphrase
is required from the user to spend coins, or to create new addresses.

Keys are encrypted with AES-256-CBC through OpenSSL's EVP library. The key is
calculated via EVP_BytesToKey using SHA512 with (by default) 25000 rounds and
a random salt.

By default, the user's wallet remains unencrypted until they call the RPC
command encryptwallet <password> or, from the GUI menu, Options->
Encrypt Wallet.

When the user is attempting to call RPC functions which require the password
to unlock the wallet, an error will be returned unless they call
walletpassword <password> <time to keep key in memory> first.

A keypoolrefill command has been added which tops up the users keypool
(requiring the password via walletpassword first).
keypoolsize has been added to the output of getinfo to show the user the
number of keys left before they need to specify their password and call
topupkeypool.

When the keypool runs out (and wallet is locked) GetOrReuseKeyFromPool
returns vchDefaultKey, meaning miners may start to generate many blocks to
vchDefaultKey instead of a new key each time.

A walletpasswordchange <oldpassword> <newpassword> has been added to allow
the user to change their password via RPC.

Whenever keying material (unencrypted private keys, the user's password, the
wallet's AES key) is stored unencrypted in memory, any reasonable attempt is
made to mlock/VirtualLock that memory before storing the keying material.
This is not true in several (commented) cases where mlock/VirtualLocking the
memory is not possible.

Although encryption of private keys in memory can be very useful on desktop
systems (as some small amount of protection against stupid viruses), on an
RPC server, the password is entered fairly insecurely. Thus, the only main
advantage encryption has for RPC servers is for RPC servers that do not spend
coins, except in rare cases, eg. a webserver of a merchant which only receives
payment except for cases of manual intervention.

Thanks to jgarzik for the original patch and sipa for all his input.