Fixes for several bugs

[gsnw-sebast]

Improved error handling, resource leaks, and robustness

• fcntl F_GETFL/F_SETFL: Check return values, propagate result
• Fixed FD leak in dgram_bind_and_get_id: call close(fd) on error
• dgram_bind_and_get_id: Option → Result<Option, io::Error>
• build_icmp_packet: kind parameter enabled, ICMPv6 RAW checksum TODO added
• recv_ping: Separate ICMP type checking for IPv4 and IPv6
• recv_ping: Distinguish EAGAIN/WouldBlock from actual errors
• parse_icmp_packet: I/O and packet parsing moved to a separate function
• ICMP offsets as named constants (no more magic numbers)
• Source IP validation: TODO comment added