Support ^U for clearing password

^U is a fairly common key combo for clearing fields, this patch adds
support for it.

https://git.suckless.org/slock/commit/bf0a5577acbd9ec7c6a577601b97144b205840d7.html
https://git.suckless.org/slock/commit/3791a996e230aec59d61faea46634ae4f031e359.html

quick cancel: allow cancelling by key press in addition to mouse movement ref. #11

simplify post-lock cmd and die if it fails

this patch does two things:

0. simplify the code by using posix_spawn()
1. unify the behavior of what happens if the post-lock cmd fails.

currently, if `fork()` fails, slock will die without locking the screen.
HOWEVER if `execvp()` fails it prints a message to stderr (which the
user cannot see since the screen has been locked already) and only exits
the child while the parent locks the screen.

to reproduce:

	# slock some_bin_that_doesnt_exist

this behavior is inconsistent, if the idea is that post-lock cmd is
_not_ important then we shouldn't `die()` on `fork()` failure either.
and if we assume that the post-lock cmd _is_ important, then we should
die on exec failure as well.

this patch assumes the latter and calls `die()` if `posix_spawn()`
fails.

ref.
https://git.suckless.org/slock/commit/a70d5d2429abf8dcb70a8817990975dc9a621d27.html

write version to stdout like the man page says

ref.
https://git.suckless.org/slock/commit/e8bca65d629a4faa89439c9f0e599efb5a259573.html

Merge pull request #8 from herrwusel/pam_consistant

Consistent unlock styles with pam and blur patches

Consistent unlock styles with pam and blur patches

Adding background image patch

Proposed change to avoid high CPU usage ref. #7

Fix for secret password patch compilation error ref. #6

Adding dwmlogo patch ref. #5

Adding auto-timeout patch

Adding secret password patch

Adding failure-command patch

Adding color message patch ref. #3

Adding alpha patch

Adding blur pixelated screen patch

Adding keypress_feedback patch

Fix bug in DPMS patch + moved patches.h to patches.def.h to match that of config

Adding xresources patch

Added capscolor, control clear, dpms, mediakeys, message, pam auth, quickcancel patches