Expose a global switch to disable IPV6

[antonfirsov]

Fixes#47583. Resolves#52287, #54807 and similar issues, without changing customer application code.

Introduces an AppContext switch System.Net.DisableIPv6 and environment variable DOTNET_SYSTEM_NET_DISABLEIPV6 to emulate the lack of OS-level IPv6 support. This is useful to workaround dual-stack socket issues when the OS reports support of IPv6, but some other underlying infrastructure element (typically VPN) doesn't function well with IPv6 request.

For consistency, this switch also impacts NameResolution and Ping, not only Sockets.

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Issue Details

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Fixes #47583. Resolves #52287, #54807 and similar issues.

Introduces an AppContext switch System.Net.DisableIPv6 and environment variable DOTNET_SYSTEM_NET_DISABLEIPV6 to emulate the lack of OS-level IPv6 support. This is useful to workaround dual-stack socket issues when the OS reports support of IPv6, but some other underlying infrastructure element (typically VPN) doesn't function well with IPv6 request.

For consistency, this switch also impacts NameResolution and Ping, not only Sockets.

Author:	antonfirsov
Assignees:	-
Labels:	area-System.Net
Milestone:	-

[antonfirsov]

Test failures are related, the IPv6 filtering doesn't work in NameResolutionPal.Unix, converting to draft. Fixed.