fix return value of system on failure to spawn child process

fix return value of system on failure to spawn child process

the value 0x7f00 (as if by _exit(127)) is specified only for the case
where the child is created but then fails to exec the shell, since
traditional fork+exec implementations do not admit reporting an error
via errno in this case without additional machinery. it's unclear
whether an implementation not subject to this failure mode needs to
emulate it; one could read the standard as requiring that. if so,
additional code will need to be added to map posix_spawn errors into
the form system is expected to return. but for now, returning -1 to
indicate an error is significantly better behavior than always
reporting failures as if the shell failed to exec after fork.