indent(1): improve handling of boxed comments indentation

The trick is to copy everything from the start of the line into the buffer
that stores newlines and comments until indent finds a brace or an else.
pr_comment() will use that information to calculate the original indentation
of the boxed comment.

This requires storing two pieces of information: the real start of the
buffer (sc_buf) and the start of the comment (save_com).