OpenSSL thread safety

freerdp/winpr had the following issues:
* The non reentrant SSL_library_init() was called concurrently (crash)
* Missing code/api to set the eventually required OpenSSL static and dynamic locking callbacks
* Missing code/api to free the application-global or thread-local OpenSSL data and tables

This commit creates two new winpr functions:

**BOOL winpr_InitializeSSL(DWORD flags):**

Use the flag WINPR_SSL_INIT_ALREADY_INITIALIZED if you want to tell winpr that your application has  already initialized OpenSSL.
If required use the flag WINPR_SSL_INIT_ENABLE_LOCKING to tell winpr that it should set the OpenSSL static and dynamic locking callbacks.
Otherwise just call it with the flag WINPR_SSL_INIT_DEFAULT.

The recommended way is that your application calls this function once before any threads are created. However, in order to support lazy OpenSSL library initialization winpr_InitializeSSL() can also safely be called multiple times and concurrently because it uses the new InitOnceExecuteOnce() function to guarantee that the initialization is only performed successfully once during the life time of the calling process.

**BOOL winpr_CleanupSSL(DWORD flags):**

If you create a thread that uses SSL you should call this function before the thread returns using the flag WINPR_SSL_CLEANUP_THREAD in order to clean up the thread-local OpenSSL data and tables.
Call the function with the flag WINPR_SSL_CLEANUP_GLOBAL before terminating your application.

**Note:** This commit only replaced the current occurences of the SSL_load_error_strings(); SSL_library_init(); pairs in the freerdp source with winpr_InitializeSSL(). None of the server or client applications has been
changed according to the recommended usage described above (TBDL).