Also switch the (unused) verification code to low-s instead of even-s.

a81cd968 introduced a malleability breaker for signatures
(using an even value for S). In e0e14e43 this was changed to
the lower of two potential values, rather than the even one.
Only the signing code was changed though, the (for now unused)
verification code wasn't adapted.
Fix script test handling of empty scripts

Previously an empty script would evaluate to OP_0
Test CHECKMULTISIG with m == 0 and n == 0
Bugfix: configure: Look in libx32 subdirectory for boost libraries
[Qt] catch Windows shutdown events while client is running

- prevents unsafe shutdowns on Windows, which is known to be
  able to cause problems with wallet.dat
- if a users ends a Windows session, this will initiate a client shutdown
  and show a Windows dialog, that tells the user what is going on (for
  Windows Vista and higher it will even show a reason for blocking the
  Windows session end)
script: add CScriptNum class

This class holds an int64_t and replaces the use of CBigInt for script
integrals.
script: switch to CScriptNum usage for scripts
script: switch outside users to CScriptNum
script: remove bignum dependency
script: add additional script tests
script: Add test for CScriptNum

Because this class replaces some usages of CBigNum, tests have been added to
verify that they function the same way. The only difference in their usage is
the handling of out-of-range numbers.

While operands are constrained to [-0x7FFFFFFF,0x7FFFFFFF], the results may
overflow. The overflowing result is technically unbounded, but in practice
it can be no bigger than the result of an operation on two operands. This
implementation limits them to the size of an int64.

CBigNum was unaware of this constraint, so it allowed for unbounded results,
which were then checked before use. CScriptNum asserts if an arithmetic
operation will overflow an int64_t, since scripts are not able to reach those
numbers anyway. Additionally, CScriptNum will throw an exception when
constructed from a vector containing more than 4 bytes This mimics the previous
CastToBigNum behavior.
Add CLIENT_VERSION_REVISION to mac osx info plist
Split up CheckBlock in a block and header version
Split AcceptBlockHeader from AcceptBlock.

Also modify some connection logic to deal with non-full blocks in the index.
devtools: add script to check symbols from Linux gitian executables

Add a script to check that the (Linux) executables produced by gitian
only contain allowed gcc, glibc and libstdc++ version symbols.  This
makes sure they are still compatible with the minimum supported Linux
distribution versions.
[Qt] Optionally add third party links to transaction context menu
Update contrib/debian for autotools, among a few other tweaks
Merge pull request #4098

6e40fcf Update contrib/debian for autotools, among a few other tweaks (Matt Corallo)
Merge pull request #4089

74fc254 devtools: add script to check symbols from Linux gitian executables (Wladimir J. van der Laan)
Merge pull request #4043

d282c1f [Qt] catch Windows shutdown events while client is running (Philip Kaufmann)