Add new event type for conflicts.

Conflicts during integrity check are now handled by special event type:
PKG_EVENT_INTEGRITYCHECK_CONFLICT.
Pkg now produces json event when find such conflict, for example:
{
    "type": "INFO_INTEGRITYCHECK_CONFLICT",
    "data": {
        "pkgname": "memcached",
        "pkgversion": "1.2.8_3",
        "pkgpath": "/usr/local/share/doc/memcached/protocol.txt",
        "conflicts": [
            "memcached-1.4.15"
        ]
    }
}
that allows scripts or side applications to detect conflicts found
during integrity check.

Add -F flag to pkg-upgrade that allows to fetch packages suitable for
upgrade and perform integrity check but skip the installation stage.