resize lockscreen window after Xrandr resize

Blank the screen with color 0, add third color for failed logins

- Adds another color in config.def.h, COLOR_INIT
- Renames the colours from numerical ones to ones with meaningful names;
  COLOR_INPUT for when there is content in the input buffer and COLOR_EMPTY
  for when the input buffer has been cleared (backspaced or a failed attempt).
- Ensures XFreeColors frees the right number of colours. This is now derived
  from the size of `Lock->colors` rather than being an integer literal.
- Makes slock exhibit the behaviour described by Markus

The default colours are the same as the ones slock currently uses, with the
exception of the new color, which I have set to red, as it indicates someone
has either failed an attempt to unlock, or that they have entered input and
erased it all.

applied Dimitris' style patch from Dec'14, with some minor modifications

fixed usage string

applied sin's patch and prepared new release

end{pw,sp}ent() can only be called after get{pw,sp}ent()

Calling them unconditionally can result in memory corruption.

Only check errno if getpwuid() fails

Checking errno otherwise is unspecified.

Add /etc/passwd support

Fix slock to work with /etc/passwd without /etc/shadow.
while we're at it, remove an occurence of trailing whitespace.

Set errno to 0 before getpwuid() and check it afterwards

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

applied andres' multi-slock fix, thanks for spotting this issue

added Ben's password placeholder entry kludge

applied Eckehard Bern's dualcolor patch to slock

select for SubstructureRedirectMask as well

fixed missing new lines

typo fix

added XRaiseWindow workaround when new clients are launched

applied Michaels patch

added dummy error handler to prevent X from terminating slock just in case

applied Markus' patch to die in case /etc/passwd is unavailable