make [p]error_msg[_and_die] more fork-in-multithreaded-app safe

Stopped using fflush(stdout) before writing to STDOUT_FILENO.
Using _exit() instead of exit().
Got rid of vasprintf/realloc: now will always use stack for the buffer.
As a bonus, memmove is gone too.

Signed-off-by: Denys Vlasenko <dvlasenk@redhat.com>