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.
Merge pull request #52 from andyrudoff/instructions

pmem: support for pmem instructions on x86
common: correct some typos and spelling errors
Merge pull request #53 from ldorau/common-correct-some-typos-and-spelling-errors

common: correct some typos and spelling errors
commmon: allow appending extra CFLAGS/LDFLAGS via command line

With this change, user may provide the extra flags CFLAGS/LDFLAGS using
the following syntax:

Example #1:
> make EXTRA_CFLAGS=-finstrument-functions <target>

Example #2:
> CFLAGS=-finstrument-functions make <target>
test: fix gitignore in pmem_isa_proc unit test
Merge pull request #54 from krzycz/common-makefile_extra_cflags

common: allow appending extra CFLAGS/LDFLAGS via command line
Merge pull request #55 from ldorau/test-fix-gitignore-in-pmem_isa_proc-unit-test

test: fix gitignore in pmem_isa_proc unit test
test: remove unintended commit of a built file
doc: link to web page content from doc README
doc: minor tweaks for HTML formatting