DEV9: Various Socket Fixes

[TheLastRar]

Description of Changes

Skip over invalid gateways for DHCP Auto Gateways on Windows
Correct SetDscpValue() function definition and remove unused duplicate definitions for Read/WriteDNSString()
Guard against out of bound reads when handling ICMP port rejected packets
Use Common functions for aligning header length
Fix possible race-condition when removing closed connections

Rationale behind Changes

A gateway of 0.0.0.0 is not usable, code to exclude this existed for other platforms.

Function definitions should not be incorrect.

When handling malformed ICMP packets, the offset to the contained IP packet was not accounted for when reading said IP packet.
Additionally, make sure we don't loop (potentially) infinitely if we get a really malformed packet.

Existing alignment for IP and TCP options was correct enough to work, but not actually correct.

If you are unlucky, a connection could close itself while the adapter was reset (potentially closing itself again),
This would result in our connection close handler trying to delete the connection twice.
Check if the connection is in our connections map, if it isn't then we are already handling it.

I opted to group these into a single PR, If it is preferred to separate then out into their own PRs, then I can do so.

Suggested Testing Steps

Test online games with Sockets