rtsold: Fix bugs reported by Coverity

• Avoid leaking a socket if llflags_get() fails.
• Avoid leaking a file handle if rtsold_init_dumpfile() fails.
• Tighten the check in if_nametosdl() which determines whether we failed to find the specified interface.
• Fix errno handling in an error path in rtsock_open().

MFC after: 1 week