Unix Pageant: support multiple fingerprint types.

The callback-function API in pageant.h for key enumeration is modified
so that we pass an array of all the available fingerprints for each
key.

In Unix Pageant, that's used by the -l option to print whichever
fingerprint the user asked for. (Unfortunately, the option name -E is
already taken, so for the moment I've called it --fptype. I may
revisit that later.)

Also, when matching a key by fingerprint, we're prepared to match
against any fingerprint type we know, with disambiguating prefixes if
necessary (e.g. you can match "md5🆎12" or "sha256:Ab12". That has
to be done a bit carefully, because we match MD5 hex fingerprints
case-insensitively, but SHA256 fingerprints are case-sensitive.

pageant.h

@@ -230,10 +230,8 @@ struct pageant_pubkey {
 struct pageant_pubkey *pageant_pubkey_copy(struct pageant_pubkey *key);
 void pageant_pubkey_free(struct pageant_pubkey *key);
 
-typedef void (*pageant_key_enum_fn_t)(void *ctx,
-                                      const char *fingerprint,
-                                      const char *comment,
-                                      uint32_t ext_flags,
+typedef void (*pageant_key_enum_fn_t)(void *ctx, char **fingerprints,
+                                      const char *comment, uint32_t ext_flags,
                                       struct pageant_pubkey *key);
 int pageant_enum_keys(pageant_key_enum_fn_t callback, void *callback_ctx,
                       char **retstr);