clear passwords with explicit_bzero

Make sure to explicitly clear memory that is used for password input. memset
is often optimized out by the compiler.

Brought to attention by the OpenBSD community, see:
https://marc.info/?t=146989502600003&r=1&w=2
Thread subject: x11/slock: clear passwords with explicit_bzero

Changes:

- explicit_bzero.c import from libressl-portable.
- Makefile: add COMPATSRC for compatibility src.
- config.mk: add separate *BSD section in config.mk to simply uncomment it on
  these platforms.

1 files changed, 2 insertions, 0 deletions

diff --git a/util.h b/util.h
new file mode 100644
index 0000000..6f748b8
--- /dev/null
+++ b/util.h
@@ -0,0 +1,2 @@
+#undef explicit_bzero
+void explicit_bzero(void *, size_t);