string: Clean up for_each_line() and related functions.

string: Clean up for_each_line() and related functions.

While for_each_complete_line() is a static function, there are two
public functions, for_each_line() and for_each_line_ro(), which are
implemented as simple wrappers for the static one. The only difference
between the two is that for_each_line_ro() sets the FELF_READ_ONLY
bit while for_each_line() does not.

Since we are about to add another bit of information soon, let's make
the bitmap and the static function public, and remove the two wrappers.
All callers are changed to pass an additional "unsigned flags" to
indicate whether they want the read-only behavior.

Documentation is updated accordingly.