plowdown: add umask, all temporary files are 600

Note: When using custom results (see --printf command-line switch), if pattern is containing %c or %C substring, then provided cookie file will have 600 permissions.
core.sh (parse_form_input_by_{name,id}): fix parsing

Previously, parsing would also match if the searched 'id'/'name'
was a substring of the actual one.

For example:

parse_form_input_by_id "<input id='abcdef' value='1' />
<input id='abc' value='2' />" 'abc'

would return '1' and not '2' as expected.