pmem2: add source get fd/handle

src/include/libpmem2.h

src/libpmem2/libpmem2.def

src/libpmem2/libpmem2.link.in

src/libpmem2/source_posix.c

src/libpmem2/source_windows.c

src/test/scope/out13.log.match

src/test/scope/out14.log.match