[uapp][dd] Fix help text for "G"/"GB" multiplier suffixes

Fix the text to match the code below, which does this:

MAYBE_MULTIPLY_SUFFIX(endptr, *out, "G", 1UL << 30);
MAYBE_MULTIPLY_SUFFIX(endptr, *out, "GB", 1000 * 1000 * 1000UL);

Bug: none
Test: none
Change-Id: I0f147ab2536388fdde37b7c102bd69559ed96668

[formatting] Convert usage of %lu to either %zu or PRIu64

Change-Id: I26618437b9e06bf4689f771847d66e4cc6cc1839

[dd] Implement basic version of dd

For simplicity, data conversion aspects of dd have
been removed.

Change-Id: Ic5fa56bb0ab81fb374282fc077e865187361d67d