lib/ini: fix bad subtring for path

Signed-off-by: Jean Privat <jean@pryen.org>
lib/stream: document and test `append_line_to`

Signed-off-by: Jean Privat <jean@pryen.org>
lib/stream: `read_line` chomp EOL, like read_lines

Signed-off-by: Jean Privat <jean@pryen.org>
lib/stream: document `read_all`

Signed-off-by: Jean Privat <jean@pryen.org>