Fail consistently with non-existing packages

Make error reporting more consistent with non-existing packages.

Before:
 1. `pkg install non-existent`
    - Displays that the "non-existent" package does not exist;
    - returns 70 (EX_SOFTWARE).
 2. `pkg install apache22 non-existent`
    - Displays that the "non-existent" package does not exist (however
      it's harder to catch it);
    - Installs apache22;
    - returns 0 (success while the result is not what we expect).

This provide consistent failure (as in 1) when some packages are not
found.

While here, add a missing line break at the end of the error message for
inexistent packages.
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.
Do not return 0 when `pkg add` fails.
Set retcode to EPKG_FATAL on failure.
Make pkg_not_found a bool instead of an int.
Fix memory leak.
Fix style.
Merge pull request #421 from smortex/no-script

Add switches to prevent running installation/deinstallation scripts
Merge pull request #422 from smortex/pkg-add-return-value

Do not return 0 when `pkg add` fails.
Merge pull request #420 from smortex/consistent-error-reporting

Fail consistently with non-existing packages
Add romain in the contributor list