Implement multi-architecture support.

This commit adds a number of new features to pkgsrc to enable
multi-architecture packages:

  - pkgtools/isaexec is a new package which integrates the isaexec
    wrapper from Solaris.  Multi-architecture binaries are hardlinks
    to this file, which then determines the architecture-dependent
    binary to run.

  - pkg_install grows "@link" support to allow isaexec hardlinks to
    be created at pkg_add time.

  - mk grows a MULTIARCH variable, set by --multiarch in bootstrap,
    which determines whether or not to build multiarch packages.

  - Each phase target will run twice (once for each architecture)
    when MULTIARCH is enabled, and the final package will combine the
    output of each build.

As for package support:

  - Packages opt-in to multi-architecture support by setting
    USE_MULTIARCH=yes, but this is only activated based on MULTIARCH.

  - BINARCHSUFFIX and LIBARCHSUFFIX will expand to directory suffixes
    for the current ABI, and will normally be used where there are
    hardcoded 'bin' or 'lib' paths.

  - When MULTIARCH is enabled, pkgsrc will build each package twice
    (once for each architecture), and then combine the two into a
    single package.

This is currently only supported on SunOS, but it is generic enough
that support can easily be added for other platforms.
Convert "archivers" category to multiarch.
Convert "audio" category to multiarch.
Convert "chat" category to multiarch.
Convert "converters" category to multiarch.
Convert "databases" category to multiarch.
Convert "devel" category to multiarch.
Convert "fonts" category to multiarch.
Convert "geography" category to multiarch.
Convert "graphics" category to multiarch.
Convert "lang" category to multiarch.
Convert "mail" category to multiarch.
Convert "math" category to multiarch.
Convert "meta-pkgs" category to multiarch.
Convert "misc" category to multiarch.
Convert "multimedia" category to multiarch.
Convert "net" category to multiarch.
Convert "pkgtools" category to multiarch.
Convert "print" category to multiarch.
Convert "security" category to multiarch.