Clean up CMerkleTx::SetMerkleBranch.

The case SetMerkleBranch(NULL) was never actually used, and thus the
involved code (loading the block from disk) can be removed and the
implementation simplified.
Add warnings for autogen.sh. Fix AC_CONFIG_SRCDIR.
Update obsolete AC macros.
qt: Change splash screen to normal window

Makes it possible to move, minimize, unminimize the window while
Bitcoin Core is initializing.
[Qt] include and file header cleanup

- alphabetical ordering
- correct ordering own headers before normal headers etc.
Apply clang-format on some infrequently-updated files
Fixing condition 'sabotaging' MSVC build
test: Fix DoS tests after c74332c

Fix data structure mismatch ... The mind boggles that they were still passing at all.
Also create pid file in non-daemon mode

Always make a pid file, not only when `-daemon` specified.

This is useful for troubleshooting, for attaching debuggers and loggers
and such.

- Write the pid file only after the datadir lock was acquired
- Don't create or remove a pid file on WIN32, and also don't show the option
Add warning about the merkle-tree algorithm duplicate txid flaw

Lots of people read the Bitcoin Core codebase to learn more about
crypto; better to warn about flaws explicitly so they don't blindly copy
the code for other uses and create broken systems.
Merge pull request #4933

20e01b1 Apply clang-format on some infrequently-updated files (Pieter Wuille)
Merge pull request #4950

33a2771 test: Fix DoS tests after c74332c (Wladimir J. van der Laan)
Fixing improper input syntax and failing bounds check
Merge pull request #4952

01c2807 Add warning about the merkle-tree algorithm duplicate txid flaw (Peter Todd)
Merge pull request #4941

a49f11d qt: Change splash screen to normal window (Wladimir J. van der Laan)
qt: Make splash and shutdown window ignore close events

It's strange to be able to close these windows while there is work
in progress.

Also set Qt::WA_DeleteOnClose on both windows to make sure that they
are deleted eventually, no matter what happens.
Merge pull request #4951

d6712db Also create pid file in non-daemon mode (Wladimir J. van der Laan)
Merge pull request #4947

6134b43 Fixing condition 'sabotaging' MSVC build (ENikS)
Merge pull request #4852

5e83bc4 [Qt] include and file header cleanup (Philip Kaufmann)
Merge pull request #4787

4b0deb3 Clean up CMerkleTx::SetMerkleBranch. (Daniel Kraft)