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.
Shouldn't we indicate here, that using these function to copy data from RAM to PM, or from RAM to RAM is not a good idea?