Add option to disable 077 umask (create new files with system default umask)

The option is only effective for either wallet-less builds or if
-disablewallet is specified as well.

This is useful when the wallet is handled by an application other than bitcoind (for example Armory). If bitcoind runs as a user different from the user that runs Armory, Armory is not able to read the blockchain files that it needs to. This allows bitcoind to be run as a separate user and function with Armory (and other applications that need to read bitcoind data files) at the same time.