Only for a child worker when not debugging.
Command line: pkg -d -- doesn't take an argument.  Fix for the
!HAVE_JAILS case.
Add new pkgdb_open_all() -- same as pkgdb_open(), but allows
specifying the pkg repository to use as the 'reponame' arg. If
'reponame' is NULL, is identical to pkgdb_open()

Similarly for pkgcli_update(), add a reponame argument which, if not
NULL, is the only repository whose catalogue will be updated.

Add '-r reponame' option to 'pkg update' allowing selective updating
of repositories.

Change the effect of '-r reponame' in 'pkg install', 'pkg upgrade',
'pkg search', 'pkg rquery' and 'pkg version' to use only the named
repository, and to use that repository even if it is marked active:no
in repo.conf.

Thus on a typically configured FreeBSD 10 box, 'pkg install -r FreeBSD pkg'
will upgrade pkg using only the package available from pkg.FreeBSd.org.

Fixes #738