Addresses code-quality issues from testsdiepraam and miDeb.

- Removes dd from feat_require_unix (keeps it in feat_common_core)
- Changes name of make_linux_iflags parameter from oflags to iflags.
- Removes roughed out SIGINFO impl.
- Renames plen -> target_len.
- Removes internal fn def for build_blocks and replaces with a call to
  chunks from std.
- Renames apply_ct to the more descriptive apply_conversion
- Replaces manual swap steps in perform_swab with call to swap from std.
- Replaces manual solution with chunks where appropriate (in Read, and Write impl).
- Renames xfer -> transfer (in local variable names).
- Improves documentation for dd_filout/dd_stdout issue.
- Removes commented debug_assert statements.
- Modifies ProdUpdate to contain ReadStat and WriteStat rather than
  copying their fields.
- Addresses verbose return in `Output<File>::new(...)`
- Resoves compiler warning when built as release when signal handler fails to
  register.
- Derives _Default_ trait on ReadStat.
- Adds comments for truncated lines in block unblock tests
- Removes `as u8` in block unblock tests.
- Removes unecessary `#[inline]`
- Delegates multiplier string parsing to uucore::parse_size.
- Renames 'unfailed' -> 'succeeded' for clairity.
- Removes #dead_code warnings. No clippy warnings on my local machine.
- Reworks signal handler to better accomodate platform-specific signals.
- Removes explicit references to "if" and "of" in dd.rs.
- Removes explicit references to "bs", "ibs", "cbs" and "status" in
  parseargs.rs.
- Removes `#[allow(deadcode)]` for OFlags, and IFlags.
- Removes spellchecker ignore from all dd files.
- Adds tests for 'traditional' and 'modern' CLI.

tests/by-util/test_dd.rs

@@ -1,5 +1,3 @@
-/* cspell:disable */
-
 use crate::common::util::*;
 
 use std::fs::{File, OpenOptions};