Add SyncDBListByDBName method (#62)

It allows to get a database list consisting of a single database.
Also added Append method for DBList required to implement the method above.

db.go

@@ -48,6 +48,16 @@ func (l DBList) Slice() []IDB {
 	return slice
 }
 
+// Append modifies a DB list by appending the given DB.
+func (l *DBList) Append(db IDB) {
+	cdblist := (*C.alpm_list_t)(unsafe.Pointer(l.list))
+	cdb := unsafe.Pointer(db.(*DB).ptr)
+
+	cdblist = C.alpm_list_add(cdblist, cdb)
+
+	l.list = (*list)(unsafe.Pointer(cdblist))
+}
+
 // SyncDBByName finds a registered database by name.
 func (h *Handle) SyncDBByName(name string) (db IDB, err error) {
 	dblist, err := h.SyncDBs()
@@ -70,6 +80,20 @@ func (h *Handle) SyncDBByName(name string) (db IDB, err error) {
 	return nil, fmt.Errorf("database %s not found", name)
 }
 
+// SyncDBListByDBName creates and returns a database list with a single
+// database given by name.
+func (h *Handle) SyncDBListByDBName(name string) (IDBList, error) {
+	db, err := h.SyncDBByName(name)
+	if err != nil {
+		return nil, err
+	}
+
+	dblist := h.NewDBList()
+	dblist.Append(db)
+
+	return dblist, nil
+}
+
 // RegisterSyncDB Loads a sync database with given name and signature check level.
 func (h *Handle) RegisterSyncDB(dbname string, siglevel SigLevel) (IDB, error) {
 	cName := C.CString(dbname)

handle.go

@@ -408,6 +408,11 @@ func (h *Handle) SyncDBs() (IDBList, error) {
 	return &DBList{(*list)(unsafe.Pointer(dblist)), *h}, nil
 }
 
+// NewDBList returns a new empty DB list.
+func (h *Handle) NewDBList() IDBList {
+	return &DBList{nil, *h}
+}
+
 func (h *Handle) CheckSpace() (bool, error) {
 	ok := C.alpm_option_get_checkspace(h.ptr)
 

interfaces.go

@@ -123,6 +123,8 @@ type IDBList interface {
 	ForEach(func(IDB) error) error
 	// Slice converts DB list to DB slice.
 	Slice() []IDB
+	// Append modifies DB list with given DB appended.
+	Append(IDB)
 	// PkgCachebyGroup returns a PackageList of packages belonging to a group
 	FindGroupPkgs(string) IPackageList
 	// FindSatisfier searches a DBList for a package that satisfies depstring