Refactored ngx_solaris_sendfilev_chain(). Though ngx_solaris_sendfilev_chain() shouldn't suffer from the problem mentioned in d1bde5c3c5d2 since currently IOV_MAX on Solaris is 16, but this follows the change from 3d5717550371 in order to make the code look similar to other systems and potentially eliminates the problem in the future.

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