How do I use mapproject to project onto a line?

1. I need to project some points onto a line, but "project" is not giving
me accurate results.
The documentation says:
"Calculations of specific great-circle and geodesic distances or for
back-azimuths or azimuths are better done using mapproject."

However, I don't see how to a project onto a line in "mapproject".

2. Also, how come reversing the end-points in "project" gives me different
results?

echo "-117.000001 33" | /usr/local/gmt32bit/bin/project -C-118/33 -E-117/33
-Lw -Fxypqrs

-117.000001 33 0.838666571653 -3.9861033073e-09 -117.000001 33.000000004

echo "-117.000001 33" | /usr/local/gmt32bit/bin/project -C-117/33 -E-118/33
-Lw -Fxypqrs

-117.000001 33 0 3.9861033073e-09 -117 33

How come p=~0.84 and not ~0.999999 in the first case?

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