bitcoin-cli: arguments privacy

The RPC command line client gained a new argument, `-stdin`

to read extra arguments from standard input, one per line until EOF/Ctrl-D.

For example:

$ echo -e "mysecretcode\n120" | src/bitcoin-cli -stdin walletpassphrase

It is recommended to use this for sensitive information such as wallet

passphrases, as command-line arguments can usually be read from the process

table by any user on the system.