cat: Improve performance on Linux (#1978)

* cat: Improve performance, especially on Linux

* cat: Don't use io::copy for splice fallback

On my MacBook Pro 2020, it is around 25% faster to not use io::copy.

* cat: Only fall back to generic copy if first splice fails

* cat: Don't double buffer stdout

* cat: Don't use experimental or-pattern syntax

* cat: Remove nix symbol use from non-Linux

Cargo.lock

@@ -1,5 +1,7 @@
 # This file is automatically @generated by Cargo.
 # It is not intended for manual editing.
+version = 3
+
 [[package]]
 name = "advapi32-sys"
 version = "0.2.0"
@@ -1581,7 +1583,8 @@ name = "uu_cat"
 version = "0.0.4"
 dependencies = [
  "clap",
- "quick-error",
+ "nix 0.20.0",
+ "thiserror",
  "unix_socket",
  "uucore",
  "uucore_procs",

src/uu/cat/Cargo.toml

@@ -16,13 +16,16 @@ path = "src/cat.rs"
 
 [dependencies]
 clap = "2.33"
-quick-error = "1.2.3"
+thiserror = "1.0"
 uucore = { version=">=0.0.7", package="uucore", path="../../uucore", features=["fs"] }
 uucore_procs = { version=">=0.0.5", package="uucore_procs", path="../../uucore_procs" }
 
 [target.'cfg(unix)'.dependencies]
 unix_socket = "0.5.0"
 
+[target.'cfg(any(target_os = "linux", target_os = "android"))'.dependencies]
+nix = "0.20"
+
 [[bin]]
 name = "cat"
 path = "src/main.rs"