tool_getparam: add --knownhosts

To allow users to specify a known hosts file that is not the default
one: ~/.ssh/known_hosts

URL: https://github.com/curl/curl/discussions/18784

docs/cmdline-opts/Makefile.inc

@@ -147,6 +147,7 @@ DPAGES = \
   keepalive-time.md \
   key-type.md \
   key.md \
+  knownhosts.md \
   krb.md \
   libcurl.md \
   limit-rate.md \

docs/cmdline-opts/knownhosts.md

@@ -0,0 +1,31 @@
+---
+c: Copyright (C) Daniel Stenberg, <daniel@haxx.se>, et al.
+SPDX-License-Identifier: curl
+Long: knownhosts
+Arg: <file>
+Protocols: SCP SFTP
+Help: Specify knownhosts path
+Category: ssh
+Added: 8.17.0
+Multi: single
+See-also:
+  - hostpubsha256
+  - hostpubmd5
+  - insecure
+  - key
+Example:
+  - --cert certificate --key here $URL
+---
+
+# `--knownhosts`
+
+When doing SCP and SFTP transfers, curl automatically checks a database
+containing identification for all hosts it has ever been used with to verify
+that the host it connects to is the same as previously. Host keys are stored
+in such a knownhosts file. By default curl uses ~/.ssh/known_hosts in the
+user's home directory.
+
+This option lets a user specify a specific file to check the host against.
+
+The known host check can be disabled with --insecure, but that makes the
+transfer insecure.