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.
What about (same logic, reduces the number of GetBoolArg calls to one for each option):
```c++
    if (GetBoolArg("-sysperms", false)) {
#ifdef ENABLE_WALLET
        if (!GetBoolArg("-disablewallet", false))
            return InitError("E....")
#endif
    } else {
        umask(077);
    }
```