Move devel/linux-c?-libelf to devel/linux-c?-elfutils-libelf to match the
package name and to be similar to upcoming ports for other elfutils
libraries.

Address some portlint warnings in the Linux infrastructure ports.

Eliminate LINUXNAME from port Makefiles.  This was just a helper variable
without special meaning outside port Makefiles but several developers have
copied it to new ports where it was then unused, apparently thinking that
it did have some special meaning.

Eliminate RPMVERSION from Linux ports and set DISTVERSIONSUFFIX instead.
This puts all version information together in the port Makefiles.

- Autodetect the installed linux_base port and use LINUX_DEFAULT only as
  a last resort.
- Construct a list of DISTFILES for every architecture (amd64, i386).
  Select one list for regular make and all lists for make makesum so linux
  ports can have one combined distinfo file.
- Set DIST_SUBDIR?=centos so there's only one copy of common distfiles.
- Use MASTER_SITES and DISTFILES groups so make fetch is a bit smarter.
- Regenerate all distinfo files.
- For linux_base-c6 (just like linux_base-c7 already does) turn bin, lib,
  lib64 and sbin into symbolic links to the same directory under usr.
  This fixes the problem where some programs/libraries exist under / on
  FreeBSD and /usr on Linux or vice versa and then depending on the order
  of search paths Linux programs may run/load FreeBSD programs/libraries
  and fail.
- Turn usr/share/icons into a symbolic link to LOCALBASE/share/icons so

Make 64 bit Linux packages the default on amd64.

Add linux-c6-libelf, needed by linux-c6-dri.