common: Add a flag for left anchored substring match to recsel.
* common/recsel.c (struct recsel_expr_s): Add field lefta. (recsel_parse_expr): Parse it. (recsel_select): Implement selection.
This flags makes it for example easy to select keys last updated from
an ldap server:
gpg --list-filter 'select=origin=ks && -^ url =~ ldap' \
-k --with-key-origin