Update CentOS base to 6.5, add userland ports

The Fedora 10 infrastructure ports have been in use since June 2009 and, while
having served a great deed, have become unsupported upstream and hence affected
by unfixed security vulnerabilities. In addition to that, many recent Linux
binaries need newer libc / stdlibc++ versions.

This commit adds the linux-c6- userland as drop-in replacement for the -f10
infrastructure, as well as upgrading the linux_base-c6 port to CentOS 6.5.

If you want to switch to linux-c6 ports, please define at /etc/make.conf:

OVERRIDE_LINUX_BASE_PORT=c6
OVERRIDE_LINUX_NONBASE_PORTS=c6

Additionally, please add the following line to /etc/sysctl.conf:

compat.linux.osrelease=2.6.18

Upgrading procedures are shown in /usr/ports/UPDATING.

This work has been inspired by Artyom Mirgorodskiy's post to emulation@ in
November 2013, using and extending mav@'s work. It has been tested extensively
and most reported issues were already fixed. Please report any additional bug
or "features" to the emulation mailing list.

Many thanks to: mav@, rene@, allanjude@, netchild@, antoine@, everyone who's
filed Issues and Pull requests on GitHub,

PR: 186820
Differential Revision: https://reviews.freebsd.org/D793
Reviewed by: allanjude, antoine, bapt, rene
Approved by: portmgr (antoine, bapt)
Approved by: koobs (mentor)
Sponsored by: Perceivon Hosting Inc.