Use dynamic buffer for displaying pager lines. Closes #3162.

Use dynamic buffer for displaying pager lines. Closes #3162.

With too small fixed-size buffers we can't color/find certain words
that span buffers. This needs to duplicate mutt_read_line with the
adjustment to leave line termination in and not support breaking
long lines using \ at EOL. Other callers may want to use this one
instead, too as we support \-escaping in too many places.