api: Add auto expand secmem feature

* src/global.c (_gcry_vcontrol): Implement control value 78.
* src/secmem.c (auto_expand): New var.
(_gcry_secmem_set_auto_expand): New.
(_gcry_secmem_malloc_internal): Act upon AUTO_EXPAND.

Backported from master. GCRYCTL_AUTO_EXPAND_SECMEM has been replaced
by its value (78) because we want to add this on the quiet so that we
do not need to claim an API update.

• GnuPG-bug-id: T3530
• Signed-off-by: Werner Koch <wk@gnupg.org>