streamrelay: Update fallback scenario

• Rework code structure using do-while(0) pattern for cleaner error handling
• Add switch_stream_source_host() function for centralized fallback logic
• Fix fallback host comparison using streq() instead of pointer comparison
• Extend fallback triggers to connection failure, socket close, and bad HTTP response
• Fix socket leaks in connect_to_stream() on setsockopt/connect failures
• Fix memory leak in HTTP host parsing by avoiding cs_strdup allocation
• Fix potential double-free by NULLing key pointers after dvbcsa_bs_key_free()
• Add http_log() helper function to prevent memory leaks in debug logging
• Add safety checks for file descriptors before close operations
• Improve code formatting and reduce duplication