Add duplication function for the ba1_t interface

Function is present in the bam header interface (bam_hdr_dup) and only
existed in the bam_copy1(dst, src) format. Hence, code using this
interface and the header interface was incoherent.

This commit adds a bam_dup1 function implemented as a call to bam_copy1
with the proper initialization to make the API's more uniform.