PROTON-596: Suppress MSVC warnings

This suppression covers:

>..\..\proton-c\src\object\object.c(232): 
   warning C4244: 'argument' : conversion from 'ssize_t' to 'int',
   possible loss of data
2>..\..\proton-c\src\object\object.c(260):
   warning C4267: 'argument' : conversion from 'size_t' to 'int',
   possible loss of data
2>..\..\proton-c\src\windows\driver.c(777):
   warning C4800: 'int' : forcing value to bool 'true' or 'false'
   (performance warning)
6>..\..\..\..\..\tests\tools\apps\c\msgr-recv.c(90): 
   warning C4996: 'sscanf': This function or variable may be unsafe.
   Consider using sscanf_s instead. 
   To disable deprecation, use _CRT_SECURE_NO_WARNINGS

This covers the majority of the warnings. A typical 64-bit compile has 172 
warnings making real problems harder to see. It also follows the
pattern used in Qpid proper.

* C4244 and C4267 could still be real problems.
  Try enabling -Wconversion in GCC if you want to be pedantic in Qpid or
  Proton.

* C4800 is probably not a risk as it's a fairly idiomatic pattern in C.

* C4996 is MSVC complaining of functions that consume buffers using
 {pointer} and not {pointer, size}. They have been the source of seg
 faults and innumerable security vulnerabilities throughout the years.

git-svn-id: https://svn.apache.org/repos/asf/qpid/proton/trunk@1601010 13f79535-47bb-0310-9956-ffa450edef68