dgram: refactor SO_RCVBUF and SO_SNDBUF methods

This commit refactors the get/set send/receive buffer size methods in the following ways:

• Use booleans instead of strings and numbers to differentiate between the send and receive buffers.
• Reduce the amount of branching and complexity in the C++ code.

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• commit message follows commit guidelines

Affected core subsystem(s)

dgram