(scm_port_line): In docstring, note first line is 0.

(scm_port_line): In docstring, note first line is 0.
(scm_set_port_line_x): In docstring, note first line is 0.
(scm_port_column): In docstring, there's no default to current input
port, and remove shared port-line @defun.
(scm_set_port_column_x): In docstring, there's no default to current
input port, note first column is 0, remove shared set-port-line!
@defun.