Refactor some network.c code to create cidr_set_masklen_internal().

 Refactor some network.c code to create cidr_set_masklen_internal().
 
 Merge several copies of "copy an inet value and adjust the mask length"
 code to create a single, conveniently C-callable function.  This function
 is exported for future use by inet SPGiST support, but it's good cleanup
 anyway since we had three slightly-different-for-no-good-reason copies.
 
 (Extracted from a larger patch, to separate new code from refactoring
 of old code)
 
 Emre Hasegeli