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

slock.1: use standard wording for options

Remove the OPTIONS section and add an EXIT STATUS section.

ref.
https://git.suckless.org/slock/commit/a34d8fb4327bbb1afd92e7527c53fcaad547a495.html

Bump to 4f04554.

Makefile: explicit_bzero.c was copied twice (GNU make gives a warning)

bump version to 1.5

Ref.
https://git.suckless.org/slock/commit/265704d73647e0d4823126bbb7ddde1d415a618d.html
https://git.suckless.org/slock/commit/4f045545a25cc02c64bfc08d27ed2ccecb962292.html

Adding background image patch

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

Specifying latest version

Adding xresources patch

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