use wipememory macro for zero buffers after crypt

Memset may be removed by compiler due to optimisations, so use secure
wipememory macro.

Copy-pasted from Libgcrypt project:
git://git.gnupg.org/libgcrypt.git
gost28147.h: add comment before constants
gost28147.h: add KEY_SIZE macro
use wipememory macro for zero key after use

See also commit 790e912.
gost28147.c: main(): use error() to check fopen()'s errors
gost28147.c: fix whitespace
gost28147.c: actualise usage string
gost28147.c: use errx() to print usage line

Unneeded help() function removed.