netxlite: call getaddrinfo and handle platform-specific oddities (#764)

This commit changes our system resolver to call getaddrinfo directly when CGO is enabled. This change allows us to:

1. obtain the CNAME easily

2. obtain the real getaddrinfo retval

3. handle platform specific oddities such as `EAI_NODATA`
returned on Android devices

See https://github.com/ooni/probe/issues/2029 and https://github.com/ooni/probe/issues/2029#issuecomment-1140258729 in particular.

See https://github.com/ooni/probe/issues/2033 for documentation regarding the desire to see `getaddrinfo`'s retval.

See https://github.com/ooni/probe/issues/2118 for possible follow-up changes.

internal/netxlite/doc.go

@@ -54,4 +54,21 @@
 //
 // Operations 1, 2, 3, and 4 are used when we perform measurements,
 // while 5 and 6 are mostly used when speaking with our backend.
+//
+// Getaddrinfo usage
+//
+// When compiled with CGO_ENABLED=1, this package will link with libc
+// and call getaddrinfo directly. While this design choice means we will
+// need to maintain more code, it also allows us to save the correct
+// getaddrinfo return value, which is hidden by the Go resolver. Also,
+// this strategy allows us to deal with the Android EAI_NODATA implementation
+// quirk (see https://github.com/ooni/probe/issues/2029).
+//
+// We currently use net.Resolver when CGO_ENABLED=0. A future version of
+// netxlite MIGHT change this and use a custom UDP resolver in such a
+// case, to avoid depending on the assumption that /etc/resolver.conf is
+// present on the target system. See https://github.com/ooni/probe/issues/2118
+// for more details regarding ongoing plans to bypass net.Resolver when
+// CGO_ENABLED=0. (If you're reading this piece of documentation and notice
+// it's not updated, please submit a pull request to update it :-).
 package netxlite