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.
