Fix agent connection in TCP mode on Windows XP

[vikman90]

Fixes
#951

As explained at the issue comment, the network API for Windows does not support the flag MSG_WAITALL in recv(). This prevents the agent from connecting to the manager in TCP mode on Windows XP.

Solution

We implemented the os_recv_waitall() function to emulate the WAITALL behavior. We changed OS_RecvSecureTCP() to use that function and replaced calls to recv() in:

• start_agent.c: Connection handshake.
• receiver.c: Incoming message handler for UNIX.
• receiver-win.c: Incoming message handler for Windows.

Tests

1. Agent connection in TCP mode.
2. Log reporting.
3. Shared configuration delivery.
4. Agent restart via Active response.

Platforms

• Windows XP SP2 x32
• Windows XP SP3 x32
• Windows Server 2016 x64
• Debian 9
• CentOS 7
• FreeBSD 11.2