Adds some support for valgrind helpers

This patch adds an option to compile freerdp in a valgrind compliant way.
The purpose is to ease memchecking when connecting with TLS. We mark bytes
retrieved from SSL_read() as plainly defined to prevent the undefined contamination.
With the patch and the option activated you get a single warning at connection
during the handshake, and nothing after.
Merge pull request #1840 from hardening/valgrind

Adds some support for valgrind helpers