pkg: clarify argument parsing

Currently pkg parses all arguments and subcommands in a single
getopt_long() loop. This results in quite a bit of accidental
complexity, especially around the -r option for the bootstrap and add
subcommands.

This also results in the undocumented behavior of all options understood
by pkg(7) being accepted anywhere in the argument list regardless of
their position. This contradicts the behavior of pkg(8) and I find it
quite unexpected.

This patch splits the argument parsing done by pkg(7) into two phases
for global options and subcommand options. This gives behavior that
matches pkg(8), accepting the subset of pkg(8) options supported by
pkg(7) in the exact same positions that pkg(8) accepts them.

This is however a somewhat breaking change, as options are no longer
accepted by pkg(7) in places they are rejected by pkg(8). For example,
pkg -f bootstrap no longer works, only pkg bootstrap -f.

The original motivation for writing this patch was investigating support
for --rootdir in pkg(7). Since pkg(8) uses -r as the short form for
both --rootdir and --repository depending on the position, I decided
that pkg(7) needed to be brought inline with pkg(8)'s argument parsing
behavior to avoid confusion.

This patch also gets rid of args_add_message (unused since 40b9f924b1)

Relnotes: yes
Reviewed by: bapt
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D49977