Add ability to select usage of IPv4/IPv6

Add ability to select usage of IPv4/IPv6 via config, repository config or command line.

When system has IPv6 local connectivity but not global one, all pkg operations involving remote site (repository) take very long time. System fallback to IPv4, but simple no-op "pkg update" spent about 3 minutes wall time.

ssh communication could be augmented via config (PKG_SSH_ARGS option), but not libfetch usage. Also, it is not always good idea to control this behavior via global config. 

This commit adds standard options "-4" and "-6" to global set of pkg options.

Also, added option USE_IP_VERSION={0|4|6} to global config and repository config.

Command line override any config files and repository option overrides global config.

Documentation updated, too.

// lev@FreeBSD.org
'will only use IPv4 for fetching' -- that's the more usual word ordering.  Similarly at line 181.