Add `-version` option to get just the version

Adds a `-version` or `--version` option to print just the version of the program for bitcoind, bitcoin-cli and bitcoin-qt (almost every program on linux supports this, I've made the mistake of calling bitcoind with --version multiple times already, before deciding to finally add it).

Also make it that `-help` can be used to display the help (as well as existing `--help`). Up to now, `-help` was the only option that didn't work with either one or two dashes.

Edit: maybe it's a good idea to add license information as well (for example, `ls` does this too) as print the versions for dependency libraries such as OpenSSL, that would fix #1930.
Just merge, don't ask ^^... ACK².