Refactor Socket::Addrinfo::Error based on os_error

[straight-shoota]

Addrinfo::Error used to have an extra ivar error_code to keep track of the error code. This patch refactors the class to use SystemError#os_error and the associated factory methods instead, which is exactly made for this purpose.
Addrinfo::Error already inherits SystemError through Socket::Error, so this doesn't change the type hierarchy.

As a result, Addrinfo::Error#value and the Addrinfo constructors receiving a plain Int32 error code are being deprecated (they keep working as delegates to the respective SystemError methods.

A necessary addition to SystemError (first commit) is the factory method .os_error_message which allows customization of the error message constructed from the os_error value (which was previously hardcoded to os_error.message). This allows incorporation of opts values into the message.

[beta-ziliani]

Nice refactor <2