pkg(7): rework the arguments handling

Rework the arguments handling around using getopt_long:

• add long option support
• add -4 and -6 support to enforce ipv4 or ipv6

While here fix a regression which occured between FreeBSD 12.1 and
FreeBSD 12.2 where pkg bootstrap -y stopped working

PR: 252270
MFC after: 2 weeks
Submitted by: evilham <contact@evilham.com>
Differential Revision: https://reviews.freebsd.org/D27860