pmem: support for pmem instructions on x86

- Clean up libpmem.3 man page: remove pmem_fence(), add pmem_has_hw_drain().
- Fence after flush is now part of pmem_drain() for better
  performance when using drain once for multiple ranges.
- Implement environment variables documented by libpmem.3.
- Combine new test cases into pmem_clflush_proc -> pmem_isa_proc.
- Comment at top of pmem.c explains ISA usage in detail.