Add optional pgp public key autoimport

Refactor `p_gpg_list_keys`
Add `/pgp autoimport` command,
it's not described in XEP-0027, but used in some clients,
such as PSI, Pidgin. It will autoimport keys received with
`/pgp sendpub`, in plain text as a message, or using features,
provided in other clients. It doesn't autoassign them, but shows
command to assign, letting user to decide.
Improve documentation for some preexisting functions
Add contact argument to `/pgp sendpub`

tests/unittests/pgp/stub_gpg.c

@@ -103,4 +103,16 @@ char*
 p_gpg_get_pubkey(const char* const keyid)
 {
     return NULL;
-}
+}
+
+gboolean
+p_gpg_is_public_key_format(const char* buffer)
+{
+    return TRUE;
+}
+
+ProfPGPKey*
+p_gpg_import_pubkey(const char* buffer)
+{
+    return NULL;
+}

tests/unittests/test_cmd_pgp.c

@@ -69,7 +69,7 @@ cmd_pgp_start_shows_message_when_no_arg_in_wintype(win_type_t wintype)
 
     will_return(connection_get_status, JABBER_CONNECTED);
 
-    expect_cons_show("You must be in a regular chat window to start PGP encryption.");
+    expect_cons_show("You must set recipient in an argument or be in a regular chat window to start PGP encryption.");
 
     gboolean result = cmd_pgp(&window, CMD_PGP, args);
     assert_true(result);