Remove double-dash parameters from mapArgs

Should be merged after pull request #4281
("Add `-version` option to get just the version #4281"),
because is changed "--help" to "-help".

Checked that grep of 'mapArgs.count("--' returned only
three places that are fixed by pull request #4281.
Ping automatically every 2 minutes (unconditionally)

... instead of after 30 minutes of no sending, for latency measurement
and keep-alive. Also, disconnect if no reply arrives within 20 minutes,
instead of 90 of inactivity (for peers supporting the 'pong' message).
small formatting, indentation and comment fixes

- contains zero code changes
Merge pull request #4170

ac14bcc small formatting, indentation and comment fixes (Philip Kaufmann)
[Qt] allow setting listen via GUI

- add DEFAULT_LISTEN in net.h and use in the code (shared
  setting between core and GUI)

Important: This makes it obvious, that we need to re-think the
settings/options handling, as GUI settings are processed before
any parameter-interaction (which is mostly important for network
stuff) in AppInit2()!
Merge pull request #4290

56b07d2 [Qt] allow setting listen via GUI (Philip Kaufmann)
small cleanup of net

- remove an unneded else in ConnectNode()
- make 0 a double and change to 0.0 in ConnectNode()
- rename strDest to pszDest in OpenNetworkConnection()
- remove an unneded call to our REF() macro in BindListenPort()
- small style cleanups and removal of unneeded new-lines
convert an if into an else if in OpenNetworkConnection()
Merge pull request #4227

634bd61 convert an if into an else if in OpenNetworkConnection() (Philip Kaufmann)
5bd6c31 small cleanup of net (Philip Kaufmann)
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.
remove unused code from getblockchaininfo()
Merge pull request #4328

f379047 remove unused code from getblockchaininfo() (Philip Kaufmann)
Add testnet DNS seed of Alex Kotenko.
Merge pull request #4330

c30329a Add testnet DNS seed of Alex Kotenko. (Andreas Schildbach)
Merge pull request #2784

f1920e8 Ping automatically every 2 minutes (unconditionally) (Pieter Wuille)
Remove -beta suffix

It conflicts with our normal naming of releases/builds. Fixes #4221.
Ignore too-long redeemScripts while loading wallet

This avoids that long redeemScripts that were grandfathered in
prevent the wallet from loading.

Fixes #4313.
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.