In preparation of D1513, allow the NetClientSession to find out if it's the "Local Client", refs #5163.

For good practice, mention all members in the affected member initializer list:
Add booelan m_IsLocalClient and u32 m_HostID to the member initializer list (to not have them undefined, even though they are not read from until they are set).
Add the strings m_GUID and m_UserName for completeness (even though the default constructor is already called for strings).
Use nullptr instead of NULL for pointers.

Comments By: Vladislav