If you want, we can rename seq_pad() to seq_pad_and_putc(). Also we can pass
both the padding character (e.g. ' ') and the trailing character (e.g. '\n')
like seq_pad_and_putc((' ' << 8) | '\n'), though I wonder someone wants to
use '\0', '\t', '\n' etc. as the padding character...

How about let that complexity wait until it's needed?  It's not like
it's that big a PITA of a patch to write, and there's a significant
chance it will *never* be needed.