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.c

@@ -1863,6 +1863,7 @@ _get_group(preference_t pref)
         return PREF_GROUP_OTR;
     case PREF_PGP_LOG:
     case PREF_PGP_SENDFILE:
+    case PREF_PGP_PUBKEY_AUTOIMPORT:
         return PREF_GROUP_PGP;
     case PREF_BOOKMARK_INVITE:
     case PREF_ROOM_LIST_CACHE:
@@ -2084,6 +2085,8 @@ _get_key(preference_t pref)
         return "log";
     case PREF_PGP_SENDFILE:
         return "sendfile";
+    case PREF_PGP_PUBKEY_AUTOIMPORT:
+        return "pgp.pubkey.autoimport";
     case PREF_TLS_CERTPATH:
         return "tls.certpath";
     case PREF_TLS_SHOW:
@@ -2215,6 +2218,7 @@ _get_default_boolean(preference_t pref)
     case PREF_STROPHE_SM_ENABLED:
     case PREF_STROPHE_SM_RESEND:
         return TRUE;
+    case PREF_PGP_PUBKEY_AUTOIMPORT:
     default:
         return FALSE;
     }

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,