gentoo: add basic use/keywords support

This bypasses the resource as a temporary measure since its broken. This just
creates defines to copy files like we do now.

manifests/keywords.pp

+define gentoo::keywords ($source="", $ensure=present) {
+    file { "gentoo_keywords_${name}":
+        target  => "/etc/portage/package.keywords/${name}",
+        source  => "${source}",
+        ensure  => $ensure,
+    }
+}

manifests/mask.pp

+define gentoo::mask ($source="", $ensure=present) {
+    file { "gentoo_mask_${name}":
+        target  => "/etc/portage/package.mask/${name}",
+        source  => "${source}",
+        ensure  => $ensure,
+    }
+}

manifests/unmask.pp

+define gentoo::unmask ($source="", $ensure=present) {
+    file { "gentoo_unmask_${name}":
+        target  => "/etc/portage/package.unmask/${name}",
+        source  => "${source}",
+        ensure  => $ensure,
+    }
+}

manifests/use.pp

+define gentoo::use ($source="", $ensure=present) {
+    file { "gentoo_use_${name}":
+        target  => "/etc/portage/package.use/${name}",
+        source  => "${source}",
+        ensure  => $ensure,
+    }
+}