Initialize command and flag fields of transactions

This commit adds initialization codes for fields which were left
uninitialized before, potentially leading to segfaults.
Add doxygen comment frame
Add the ws_socket type
Add ws_socket_init()'s definition
Add socket_build_connection_cb()'s implementation
Add ws_socket_init()'s implementation
Do not reset command count if there are no commands

Prevents another potential segfault.
Merge pull request #526 from matthiasbeyer/add-unix_socket

Add unix socket
Merge pull request #543 from neithernut/fix-transaction-init

Initialize command and flag fields of transactions