Prepare to not localize private function descriptions

The overwhelming majority of localizable messages comes from
completions:

	$ ls share/completions/ | wc -l
	$ 1048

OTOH functions also contribute a small amount, mostly via their
descriptions (so usually just one per file).

	$ ls share/functions/ | wc -l
	$ 237

Most of these are private and almost never shown to the user, so it's
not worth bothering translators with them. So:

- Skip private (see the parent commit) and deprecated functions.
- Skip wrapper functions like grep (where the translation seems to
  be provided by apropos), and even the English description is not
  helpful.
  - Assume that most real systems have "seq", "realpath" etc.,
    so it's no use providing our own translations for our fallbacks.
- Mark fish's own functions as tier1, and some barely-used functiosn
  and completions as tier3, so we can order them that way in
  po/*.po. Most translators should only look at tier1 and tier2.
  In future we could disable localization for tier3.

See the explanation at the bottom of
tests/checks/message-localization-tier-is-declared.fish

Part of #11833

share/functions/__fish_complete_ftp.fish

@@ -1,4 +1,5 @@
-function __fish_complete_ftp -d 'Complete ftp, pftp' --argument-names ftp
+# localization: tier3
+function __fish_complete_ftp --argument-names ftp
     # Common across all ftp implementations
     complete -c $ftp -xa "(__fish_print_hostnames)" -d Hostname
     complete -c $ftp -s 4 -d 'Use IPv4 only'