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.

add slock.1 man page

prepare 1.3 release

resize lockscreen window after Xrandr resize

applied sin's patch and prepared new release

applied Robert Schneider's Linux suggestions, also bumped version and updated LICENSE file's copyright notice

prepared 1.1

applied Eckehard Bern's dualcolor patch to slock

applied Ali Gholami Rudi's patch regarding DPMS timeout customization and persistence

prepared release

fixed DPMS crashing issue

applied two patches, BSD_AUTH patch and Gottox' DPMS support patch

next version will be 0.8

final fixes and cleanups

updating copyright stuff in slock as well

grab on the root window, it is correct, all lockers do that

made slock more aggressive, DenisG please recheck

fixed a potential buffer overflow bug on the stack (thanks to Ghassan Misherg)

thanks to Jack for this hint

small fix of Vincent's patch