closures: Create temporary file with O_TMPFILE and O_CLOEXEC when available

The open_temp_exec_file_dir function can create a temporary file without
file system accessible link. If the O_TMPFILE flag is not defined (old
Linux kernel or libc) the behavior is unchanged.

The open_temp_exec_file_name function now need a new argument "flags"
(like O_CLOEXEC) used for temporary file creation.

The O_TMPFILE flag allow temporary file creation without race condition.
This feature/fix prevent another process to access the (future)
executable file from the file system.

The O_CLOEXEC flag automatically close the temporary file for any
execve. This avoid transmitting (executable) file descriptor to a child
process.
closures: Check for mkostemp(3)
Merge pull request #120 from l0kod/tmpfile

Create temporary file with O_TMPFILE and O_CLOEXEC when available