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`

src/config/preferences.h

@@ -147,6 +147,7 @@ typedef enum {
     PREF_TITLEBAR_MUC_TITLE_NAME,
     PREF_PGP_LOG,
     PREF_PGP_SENDFILE,
+    PREF_PGP_PUBKEY_AUTOIMPORT,
     PREF_TLS_CERTPATH,
     PREF_TLS_SHOW,
     PREF_LASTACTIVITY,