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/pgp/gpg.h

@@ -74,6 +74,8 @@ char* p_gpg_autocomplete_key(const char* const search_str, gboolean previous, vo
 void p_gpg_autocomplete_key_reset(void);
 char* p_gpg_format_fp_str(char* fp);
 char* p_gpg_get_pubkey(const char* const keyid);
+gboolean p_gpg_is_public_key_format(const char* buffer);
+ProfPGPKey* p_gpg_import_pubkey(const char* buffer);
 
 ProfPGPKey* p_gpg_key_new(void);
 void p_gpg_free_key(ProfPGPKey* key);