Implement `pkg delete -D`.

pkg_delete(8) featured a `-D` flag to avoid running deinstallation
scripts.  Implement this for pkg(8).
Fix indentation after last commit.
Implement `pkg add -I`, `pkg install -I` and `pkg upgrade -I`.

pkg_add(8) featured a `-I` flag to avoid running installation scripts.
Implement this for pkg(8).

For pkg(8), this flag is also valid for upgrades.  In this case,
deinstallation scripts are not run as well during the upgrade.
Fix indentation after last commit.
Fix style.
Merge pull request #421 from smortex/no-script

Add switches to prevent running installation/deinstallation scripts