No longer use a temporary buffer in the die() function

No longer use a temporary buffer in the die() function

Before the following change

  f947d0a Breaks configfiles! Major refactoring of i3status, see below

The die(fmt, ...) function was outputting the reason to the status bar
in addition to stderr. For this reason, it was meaningful to create a
temporary string according to the format string and then passing it
around to the different functions.

Nowadays, we only display the error message to stderr so calling
fprintf(stderr, ...) is much simpler.

Signed-off-by: Olivier Gayot <olivier.gayot@sigexec.com>