gpg: New option --import-filter

* g10/gpg.c (oImportFilter): New.
(opts): Add --import-filter.
(main): Handle option.
* g10/import.c: Include recsel.h, init.h, and mbox-util.h.
(import_keep_uid): New global var.
(cleanup_import_globals): New.
(parse_and_set_import_filter): New.
(filter_getval): New.
(apply_keep_uid_filter): New.
(import_one): Apply filter if set.
--

Funny new option.  It can for example be used to export a key with
only one user id:

  gpg --no-options --import --import-options import-export \
      --import-filter keep-uid='mbox=wk@gnupg.org'         \
     < full-key.pub > key-with-one-uid.pub

More features will eventually be added.

Signed-off-by: Werner Koch <wk@gnupg.org>

doc/gpg.texi

@@ -2218,6 +2218,45 @@ opposite meaning. The options are:
   Defaults to no.
 @end table
 
+@item --import-filter @code{@var{name}=@var{expr}}
+@opindex import-filter
+This option defines an import filter which is implied to the imported
+keyblock right before it will be stored.  @var{name} defines the type
+of filter to use, @var{expr} the expression to evaluate.  The option
+can be used several times which then appends more expression to the
+same @var{name}.
+
+@noindent
+The available filter types are:
+
+@table @asis
+
+  @item keep-uid
+  This filter will keep a user id packet and its dependent packets in
+  the keyblock if the expression evaluates to true.
+
+@end table
+
+The syntax for the expression is defined in the appendix (FIXME).  The
+property names for the expressions depend on the actual filter type
+and are indicated in the following table.
+
+The available properties are:
+
+@table @asis
+
+  @item uid
+  A string with the user id.  (keep-uid)
+
+  @item mbox
+  The addr-spec part of a user id with mailbox or the empty string.
+  (keep-uid)
+
+  @item primary
+  Boolean indicating whether the user id is the primary one.  (keep-uid)
+
+@end table
+
 @item --export-options @code{parameters}
 @opindex export-options
 This is a space or comma delimited string that gives options for