hpkg format: Add attribute for declaring post install scripts
packagefs: CachingPackageReader: Don't delete cached heap reader

... in destructor. After we create it, it belongs to the super class and
is destroyed by it.
WriterImplBase: Add _AddStringAttributeList()

... and simplify some code by using it.
package info parser: improve parse error column numbers

Assume 4 column tab stops and compute the column numbers accordingly.
package info parser: allow escaping of new lines

So we can break long lines without changing the semantics.