solar/pssolar: Reject -T arguments that aren't d/c/n/a letters

[Esteban82]

Description of proposed changes

-T's date must be given with the +d prefix: -Tdcna[+d<date>][+z<TZ>]. If +d is forgotten and a date is glued directly after the type letters (e.g. -Td2020-01-01T00:00:00 instead of -Td+d2020-01-01T00:00:00), GMT doesn't error - it silently ignores the leftover text and plots today's date instead of the one requested.

Found while investigating gh#9220: that issue's own reproduction script has this exact typo, so it never actually pinned the date to 2020-01-01 (it always plotted "now" - the TZ-offset comparison in that issue is still valid regardless, since it doesn't depend on which day it runs). This same mistake is easy to make since, unlike -I, the d right after -T is the terminator-type selector, not the start of the date.

This PR makes -T reject any character in its argument that isn't d, c, n, or a, so this typo now produces a clear parse error instead of silently plotting the wrong day.

Reproduction

gmt begin BUG_MissingPlusD png
	gmt basemap -JH14c -Baf -Rg
	gmt solar -Td2020-12-21T12:00:00 -W2p,blue
	gmt solar -Td2020-06-21T12:00:00 -W1p,red,-
gmt end

December 21 and June 21 are six months apart (opposite solstices) and should produce near-mirrored terminators, but both silently plot "now" instead, so the two lines land exactly on top of each other (screenshot attached separately to the requester).

Reminders

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