If the re-exec fails, be sure to remove a temp makefile that was
created to read from stdin.
* src/job.c (exec_command): Return on failure.
(child_execute_job): Call exit if exec_command returns.
* src/job.h (exec_command): Don't mark as NORETURN.
* src/main.c (main): Unlink stdin temporary file if re-exec fails.
* tests/run_make_tests.pl: Get value for ERR_nonexe_file/ERR_exe_dir.
* tests/scripts/features/temp_stdin: Test that temp file unlink works.
