Allow re-mounting an existing mount with "remount"

Without this line of code, if a volume is present in /proc/mounts,
it cannot be remounted with new mount options.

Docker-DCO-1.1-Signed-off-by: Peter Waller <p@pwaller.net> (github: pwaller)
Merge pull request #6669 from pwaller/issue-6652

Allow re-mounting an existing mount with "remount"
Merge pull request #6664 from LK4D4/bench_on_truncindex_delete

Add benchmarks on TrucnIndex.Delete
Merge pull request #6661 from tiborvass/tar_untar_file

Improve untar when using files instead of directories. Specifies behavior on non-existant targets.
Merge pull request #6646 from tiborvass/fix-workdir

Fix --workdir=/
fix TestParseRunVolumes with Go 1.3 randomization

Docker-DCO-1.1-Signed-off-by: Cristian Staretu <cristian.staretu@gmail.com> (github: unclejack)
Merge pull request #6665 from leeplay/patch-1

Update dockervolumes.md
Merge pull request #6649 from vieux/fix_api_return_codes_start_stop

return 304 is status isn't modified in start and stop
Merge pull request #6185 from jbeda/debian-init-fix

Make debian init work when called repeatedly in quick succession
Merge pull request #6639 from leeplay/userguide

Update dockervolumes.md
add Docker version and branch to html header

Docker-DCO-1.1-Signed-off-by: Sven Dowideit <SvenDowideit@docker.com> (github: SvenDowideit)
Merge pull request #6136 from thatsamguy/add-mageia-mkimage

Add mkimage support for mageia using urpmi
Merge pull request #6590 from robertbachmann/patch

Windows needs hardware virtualization
Merge pull request #6626 from SvenDowideit/pr_out_updated_cloud_sdk_download_link_added_a_command_to_set_the_project

Updated Cloud SDK download link, added a command to set the project
Merge pull request #6218 from vieux/update_maintainers
Merge pull request #6635 from SvenDowideit/deputise-fredlf
Merge pull request #6668 from erikh/fix-links-userguide

Add ticks to docker links userguide, was not being rendered properly.
Add missing parenthesis in docs for -author switch

Docker-DCO-1.1-Signed-off-by: Trent Ogren <tedwardo2@gmail.com> (github: misfo)

Docker-DCO-1.1-Signed-off-by: Trent Ogren <tedwardo2@gmail.com> (github: SvenDowideit)
Add the missing paren to the documentation for docker commit too

Docker-DCO-1.1-Signed-off-by: Sven Dowideit <SvenDowideit@docker.com> (github: SvenDowideit)
Update contrib/mkimage/debootstrap to remove /var/lib/apt/lists (trimming at least 40MB and forcing "apt-get update" in dependent images before packages can be installed)

Docker-DCO-1.1-Signed-off-by: Andrew Page <admwiggin@gmail.com> (github: tianon)