[Toybox] [PATCH] xxd: better error message.

[enh]

`xxd -r` with `-p` format input (rather than the `xxd -r -p` that
you actually need to decode that) doesn't work because the input
isn't in the expected

    <address/offset> : <byte...>

format. "real" xxd silently outputs nothing. toybox complains about
an invalid seek.

This patch outputs a clear error if we don't see the leading
address/offset and either ' ' or ':' as a separator (and tests for
both kinds of input).

Looking at the other use of scanf() in this code and testing against
"real" xxd, the fact that toybox allows `-r -i` to parse input
without commas actually matches (undocumented) xxd behavior, so
I've just added a test for that.

Bug: https://github.com/landley/toybox/issues/452
---
 tests/xxd.test   | 8 ++++++++
 toys/other/xxd.c | 8 ++++++--
 2 files changed, 14 insertions(+), 2 deletions(-)
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