[Toybox] [PATCH] host: cope with large DNS responses.

[enh]

DNS responses were limited to 512 bytes back when they were UDP only,
but if you have a TCP connection you can get up to 64KiB.

Also use the existing constant for the size of rrname.

Also update the help text.

Fixes #56.

Test: `toybox host value.testing.express`
---
 toys/pending/host.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)
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