import: rename keyring extension from .gpg to .pgp

The format is not specific to gpg, it's defined by the pgp
protocols. Lintian even warns about it:

systemd-container: openpgp-file-has-implementation-specific-extension [usr/lib/systemd/import-pubring.gpg]

Rename it accordingly, as new pgp implementations are slowly becoming
more popular.

Keep using /etc/systemd/import-pubring.gpg for backward compatibility.

src/import/meson.build

@@ -109,6 +109,6 @@ install_data('org.freedesktop.import1.service',
 install_data('org.freedesktop.import1.policy',
              install_dir : polkitpolicydir)
 
-install_data('import-pubring.gpg',
+install_data('import-pubring.pgp',
              install_dir : libexecdir)
 # TODO: shouldn't this be in pkgdatadir?

src/import/pull-common.c

@@ -458,6 +458,8 @@ static int verify_gpg(
                  * otherwise. */
                 if (access(USER_KEYRING_PATH, F_OK) >= 0)
                         cmd[k++] = "--keyring=" USER_KEYRING_PATH;
+                else if (access(USER_KEYRING_PATH_LEGACY, F_OK) >= 0)
+                        cmd[k++] = "--keyring=" USER_KEYRING_PATH_LEGACY;
                 else
                         cmd[k++] = "--keyring=" VENDOR_KEYRING_PATH;