dd: splice() fast-path for skip= with non-seekable input

[oech3]

$ hyperfine "head -c 2000000000 /dev/zero|target/release/dd-splice-skip skip=200000000" "head -c 2000000000 /dev/zero|target/release/dd skip=200000000"
Benchmark 1: head -c 2000000000 /dev/zero|target/release/dd-splice-skip skip=200000000
  Time (mean ± σ):     828.9 ms ±   9.9 ms    [User: 554.4 ms, System: 690.8 ms]
  Range (min … max):   818.5 ms … 844.2 ms    10 runs
 
Benchmark 2: head -c 2000000000 /dev/zero|target/release/dd skip=200000000
  Time (mean ± σ):      3.056 s ±  0.030 s    [User: 2.167 s, System: 2.361 s]
  Range (min … max):    3.017 s …  3.093 s    10 runs
 
Summary
  head -c 2000000000 /dev/zero|target/release/dd-splice-skip skip=200000000 ran
    3.69 ± 0.06 times faster than head -c 2000000000 /dev/zero|target/release/dd skip=200000000

[github-actions]

GNU testsuite comparison:

GNU test failed: tests/dd/no-allocate. tests/dd/no-allocate is passing on 'main'. Maybe you have to rebase?
GNU test failed: tests/misc/yes. tests/misc/yes is passing on 'main'. Maybe you have to rebase?
Skip an intermittent issue tests/tty/tty-eof (fails in this run but passes in the 'main' branch)
Note: The gnu test tests/basenc/bounded-memory is now being skipped but was previously passing.
Note: The gnu test tests/cut/bounded-memory is now being skipped but was previously passing.
Note: The gnu test tests/printf/printf-surprise is now being skipped but was previously passing.