pmem2: add pmem2_map_from_existing function

doc/Makefile

doc/libpmem2/.gitignore

doc/libpmem2/pmem2_map_delete.3.md

doc/libpmem2/pmem2_map_from_existing.3.md

src/PMDK.sln

src/include/libpmem2.h

src/libpmem2/libpmem2.def

src/libpmem2/libpmem2.link.in

src/libpmem2/map.c

src/libpmem2/map_posix.c

src/libpmem2/map_windows.c

src/test/Makefile

src/test/pmem2_integration/TESTS.py

src/test/pmem2_integration/pmem2_integration.c

src/test/pmem2_map_from_existing/.gitignore

src/test/pmem2_map_from_existing/Makefile

src/test/pmem2_map_from_existing/TESTS.py

src/test/pmem2_map_from_existing/pmem2_map_from_existing.c

...t/pmem2_map_from_existing/pmem2_map_from_existing.vcxproj

...map_from_existing/pmem2_map_from_existing.vcxproj.filters