server: Remove command pointer from struct command_context.

server: Remove command pointer from struct command_context.

Command handlers should not know about this implementation detail.

This commit also changes parse_sb_command() to not only parse the
command line but to actually run the command in case the caller has
sufficient permissions. The function is renamed to run_command()
to reflect this change.

We use the opportunity to clean up the allocation and freeing of
the command buffer and the argument vector. Both are now freed in
the same function they were allocated, which is considered good
programming practice.