curl: add long option '--out-null'

Add a new commandline option --out-null that discards all
response bytes into the void. Replaces non-portable use of
'-o /dev/null' with more efficiency.

Feature added in 8.16.0

Closes #17800

docs/cmdline-opts/Makefile.inc

@@ -181,6 +181,7 @@ DPAGES = \
   ntlm.md \
   oauth2-bearer.md \
   output-dir.md \
+  out-null.md \
   output.md \
   parallel-immediate.md \
   parallel-max.md \

docs/cmdline-opts/out-null.md

@@ -0,0 +1,26 @@
+---
+c: Copyright (C) Daniel Stenberg, <daniel@haxx.se>, et al.
+SPDX-License-Identifier: curl
+Long: out-null
+Help: Discard response data into the void
+Category: output
+Added: 8.16.0
+Multi: per-URL
+See-also:
+  - output
+  - remote-name
+  - remote-name-all
+  - remote-header-name
+Example:
+  - "https://example.com" --out-null
+---
+
+# `--out-null`
+
+Discard all response output of a transfer silently. This is the more
+efficient and portable version of
+
+    curl https://host.example -o /dev/null
+
+The transfer is done in full, all data is received and checked, but
+the bytes are not written anywhere.

docs/cmdline-opts/output.md

@@ -9,6 +9,7 @@ Category: important output
 Added: 4.0
 Multi: per-URL
 See-also:
+  - out-null
   - remote-name
   - remote-name-all
   - remote-header-name
@@ -56,6 +57,10 @@ Or for Windows:
 
     curl example.com -o nul
 
+Or, even more efficient and portable, use
+
+    curl example.com --out-null
+
 Specify the filename as single minus to force the output to stdout, to
 override curl's internal binary output in terminal prevention:
 

docs/options-in-versions

@@ -145,6 +145,7 @@
 --ntlm                               7.10.6
 --ntlm-wb                            7.22.0
 --oauth2-bearer                      7.33.0
+--out-null                           8.16.0
 --output (-o)                        4.0
 --output-dir                         7.73.0
 --parallel (-Z)                      7.66.0