[Crash-utility] [PATCH 1/2] Optimize mkstring()

A few simple changes here: - call memset() instead of repeatedly calling strcat() on the target string - use shift_string_right() for CENTER (same is already used for right RJUST) - use memset() in shift_string_right() - remove the (no longer needed) local buffer, which makes it possible to call mkstring() on an arbitrarily long string Signed-off-by: Petr Tesarik <ptesarik(a)suse.cz&gt;