MFC:

• fix harmless compiler's warnings;
• before refusing to delete package because "there are packages installed that require this package" check that packages in question is actually installed;
• add new `-r' option to pkg_delete(8), which instructs it to delete not only packages specified at command line, but all packages that depend on specified packages as well.

This brings in the following deltas:

1.57      src/usr.sbin/pkg_install/create/perform.c
1.7       src/usr.sbin/pkg_install/delete/delete.h
1.23      src/usr.sbin/pkg_install/delete/main.c
1.33      src/usr.sbin/pkg_install/delete/perform.c
1.27      src/usr.sbin/pkg_install/delete/pkg_delete.1
1.42      src/usr.sbin/pkg_install/info/perform.c
1.4       src/usr.sbin/pkg_install/lib/deps.c
1.38      src/usr.sbin/pkg_install/lib/lib.h