Add option to update ebuilds forcibly (--force?)

[bor]

There are cases when we need to update all generated ebuilds.

1. perl was updated, so corelist was changed, need to update deps, etc
2. gentoo policy, for example min requiring for EAPI was updated (gentoo #523458)

New option: --force? --update-force? --force-update?

Workarounds:

source /etc/make.conf && rm -r $GCPAN_OVERLAY/perl-gcpan/* && g-cpan -gu `qlist -I perl-gcpan/ | sed 's|perl-gcpan/||'`

source /etc/make.conf && find $GCPAN_OVERLAY/perl-gcpan -name '*.ebuild' -exec rm '{}' + && g-cpan -gu