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.

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.
util: Add function FormatParagraph to format paragraph to fixed-width

This is to be used for the `-version` and `-help` messages.
Add 'about' information to `-version` output

Adds a copyright and attribution message to the `-version` output
(the same as shown in the About dialog in the GUI).

Move the message to a function LicenseInfo in init.cpp.
qt: Unify AboutDialog and HelpMessageDialog

They share so much code and functionality that they may as well
be one class.
Merge pull request #4281

5c97aae qt: Unify AboutDialog and HelpMessageDialog (Wladimir J. van der Laan)
45615af Add 'about' information to `-version` output (Wladimir J. van der Laan)
97789d3 util: Add function FormatParagraph to format paragraph to fixed-width (Wladimir J. van der Laan)
96b733e Add `-version` option to get just the version (Wladimir J. van der Laan)