net: create resolver semaphore channels eagerly

[serhiy-bzhezytskyy]

A lookup from outside a testing/synctest bubble, after net's resolver has been used inside one, aborts with "send on synctest channel from outside bubble". One test order runs to completion and another aborts the process, and a fatal error reports no failing test.

resolverConfig.ch and nsswitchConfig.ch are created inside their initOnce.Do, so each is associated with the bubble of the first caller to reach the resolver.

Both are used only as one-slot semaphores, so they are now created in their package-level var initializers, where no bubble exists. That removes the ordering dependence at no cost.

threadLimit in net.go has the same shape but is left alone: its capacity derives from rlim.Cur, and a program may lower RLIMIT_NOFILE after package initialization, so eager creation could permit more concurrent getaddrinfo calls than there are descriptors.

Updates #80903

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