ftp/tnftp: Adopt/Update to 20230409

ChangeLog:

• Release as "tnftp 20230409".
• Use better terminology.
• Merge NetBSD ftp from 20210826 to 20230226:
  • Add option sslnoverify to control validation of SSL

	  certificates.

• Add netrc processing to fetch-mode (URL on command line) to

	  enable options and autologin via netrc.

• Fix SSL cleanup in some error paths.
• Support SSL certificate validation by default.

	  FTPSSLNOVERIFY=1 in the environment to disable validation.

• Handle relative URLs.
• Improve ftp(1) markup.
• Fix -? in a more portable manner.
• Equivalent to "NetBSD-ftp 20230225" with documentation and

	  portability fixes.