[Toybox] [CLEANUP] more dd stuff.

[Rob Landley]

This dd cleanup has turned into a "rewrite and port over" sort of thing,
so it's not quite the same incremental series of patches as a lot of
previous cleanups.

I got summary() in (using the new clock_gettime() stuff since the
milisecond time querying is apparently deprecated now), and hooked up
the signal handling (with SA_RESTART for SIGUSR1 because although Linux
no longer gives us length 0 reads with EINTR from SIGSTOP/SIGCONT, I
suspect we _can_ get them from signal handlers we install unless we
request the restart).

Then I started doing seek and skip, and that has sharp edges in what
posix says. I'm trying to follow posix closely enough that I at least
document and justify all my deviations from it, and according to posix
if lseek doesn't work you read the rest of the file that's there and
write zero padding. In the case of an output file, you may have to read
past all the existing data _and_ write zeroes to extend the file before
the normal dd can commence.

I _think_ that lseek(SEEK_SET) will return error and not move the file
pointer, or return the position you asked for, and not return some
_OTHER_ position (such as the end of the file if you seek past the end
on a filesystem that doesn't support sparse). But I need to confirm/test
this.

What busybox does is support the fallback to reading past data for
input, but for output it's lseek or nothing with no fallback to skipping
what's there and padding the output manually. Which I object to because
it's not _consistent_: if you're gonna do one...

In linux, the input/output can be a pipe (no seek at all) or a file
(seek and sparse). I should also test with char and block devices; block
is seekable but not extensible so we can test some error paths. But
there are also some fun questions about "what if the file is on a vfat
filesystem, does that support sparse files?" To which the answer is "I
dunno, let's find out"...

(What I'm working on is a design where lseek will zero out seek= or
skip= if successful, otherwise the main loop will handle it. Avoiding
duplicate code while still handling all the correct corner cases. But
I'll have to continue in the morning because I'm falling asleep right
now...)

Rob