Make strace_popen abort on error

Make strace_popen abort on error

It makes no sense to postpone abort on strace_popen error
unti it returns. Moreover, out-of-memory error was exiting
without any message.
While at it, use 0 as "none" for popen_pid, as optimization.

* strace: Initialize popen_pid to 0 - this puts it in bss.
  (trace): Reset popen_pid to 0 instead of -1.
  (strace_popen): Never return NULL as error indicator,
  abort with good error message instead.
  (main): Remove NULL check of strace_popen result.

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