Be more consistent about bracketing URLs in commentary.

Use angle brackets unless the URL is already surrounded by white space.
(Thanks to a suggestion from Steffen Nurpmeso.)
Distinguish hyphen from minus from dashes.

Most of this patch is to commentary, to distinguish better among
the similar characters hyphen '-', minus sign '−', en dash '–' (used
for ranges), and em dash '—'.  Render ordinary dashes in text as
spaced en dashes, as this gives better-looking results in monospaced
fonts.
* yearistype.sh, zic.c: Reword diagnostics so that they do not use
hyphens as if they were dashes.  Since we are sticking to ASCII in
programs, we can't use proper dashes in diagnostics.