string: Replace the for_each_line_modes enum by a bitmap.

Currently we pass an "enum for_each_line_modes" as the first argument
to for_each_complete_line(). The parameter is effectively a boolean
since the enumeration defines only two possible values.

For the upcoming discard feature of for_each_line_modes() we'll need
to pass another bit of information, so this patch changes the type of
the first argument to for_each_complete_line() to unsigned and treats
it as a bit array. The only bit defined so far is LINE_MODE_RO, with
unchanged semantics.

The two callers are updated to reflect this change.