common: Add gnupg_memstr to repalce static versions.
* common/stringhelp.c (gnupg_memstr): New. * common/mbox-util.c (my_memstr): Remove. (is_valid_mailbox_mem): Use gnupg_memstr. * common/recsel.c (my_memstr): Remove. (recsel_select): Use gnupg_memstr.