Merge Linux ncurses-base into linux_base.  It's needed by ncurses-libs.

Update Linux CentOS 7 ports to 7.4.1708.

- 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

Update Linux CentOS 7 ports to 7.3.1611.  While 7.2 had semi-official i386
packages there are no such packages for 7.3 yet so i386 support has been
marked IGNORE for now.

Change DIST_SUBDIR to just "centos" so distfiles that remain the same
between major versions don't have to be redownloaded.

Remove fmake hacks from linux.mk.

Submitted by:	Piotr Kubaj <pkubaj@anongoth.pl>

Add Linux CentOS 7 infrastructure ports.

Mk/Uses/linux.mk changes:
- Add support for architecture neutral (noarch) distfiles.
- Add support for 64-bit only ports: set IGNORE on i386 and don't install
  32-bit compat libraries on amd64.

Submitted by:	Piotr Kubaj <pkubaj@anongoth.pl>
Differential Revision:	https://reviews.freebsd.org/D7886