syscallcompat: Openat: always set O_CLOEXEC

Let's not leak fds to logger.

Before:

	$ lsof -p $(pgrep logger)
	COMMAND    PID  USER   FD   TYPE             DEVICE  SIZE/OFF    NODE NAME
	logger  146410 jakob  cwd    DIR              253,0      4096       2 /
	logger  146410 jakob  rtd    DIR              253,0      4096       2 /
	logger  146410 jakob  txt    REG              253,0     41560 6293858 /usr/bin/logger
	logger  146410 jakob  mem    REG              253,0 229754784 6292695 /usr/lib/locale/locale-archive
	logger  146410 jakob  mem    REG              253,0    186480 6292031 /usr/lib64/libgcc_s-14-20240508.so.1
	logger  146410 jakob  mem    REG              253,0    787128 6294119 /usr/lib64/libzstd.so.1.5.6
	logger  146410 jakob  mem    REG              253,0    211424 6294587 /usr/lib64/liblzma.so.5.4.6
	logger  146410 jakob  mem    REG              253,0    131128 6302636 /usr/lib64/liblz4.so.1.9.4
	logger  146410 jakob  mem    REG              253,0     49184 6302330 /usr/lib64/libcap.so.2.69
	logger  146410 jakob  mem    REG              253,0   2476880 6295299 /usr/lib64/libc.so.6
	logger  146410 jakob  mem    REG              253,0    987256 6292058 /usr/lib64/libsystemd.so.0.38.0
	logger  146410 jakob  mem    REG              253,0    906256 6295295 /usr/lib64/ld-linux-x86-64.so.2
	logger  146410 jakob    0r  FIFO               0,14       0t0  607727 pipe
	logger  146410 jakob    1w   CHR                1,3       0t0       4 /dev/null
	logger  146410 jakob    2w   CHR                1,3       0t0       4 /dev/null
	logger  146410 jakob    3u  unix 0x0000000046d9c96b       0t0  607729 type=DGRAM (CONNECTED)
	logger  146410 jakob   10u   DIR               0,33        80    7758 /tmp/tmp.lbUiEw9P6W/a

After:

	$ lsof -p $(pgrep logger)
	COMMAND    PID  USER   FD   TYPE             DEVICE  SIZE/OFF    NODE NAME
	logger  147982 jakob  cwd    DIR              253,0      4096       2 /
	logger  147982 jakob  rtd    DIR              253,0      4096       2 /
	logger  147982 jakob  txt    REG              253,0     41560 6293858 /usr/bin/logger
	logger  147982 jakob  mem    REG              253,0 229754784 6292695 /usr/lib/locale/locale-archive
	logger  147982 jakob  mem    REG              253,0    186480 6292031 /usr/lib64/libgcc_s-14-20240508.so.1
	logger  147982 jakob  mem    REG              253,0    787128 6294119 /usr/lib64/libzstd.so.1.5.6
	logger  147982 jakob  mem    REG              253,0    211424 6294587 /usr/lib64/liblzma.so.5.4.6
	logger  147982 jakob  mem    REG              253,0    131128 6302636 /usr/lib64/liblz4.so.1.9.4
	logger  147982 jakob  mem    REG              253,0     49184 6302330 /usr/lib64/libcap.so.2.69
	logger  147982 jakob  mem    REG              253,0   2476880 6295299 /usr/lib64/libc.so.6
	logger  147982 jakob  mem    REG              253,0    987256 6292058 /usr/lib64/libsystemd.so.0.38.0
	logger  147982 jakob  mem    REG              253,0    906256 6295295 /usr/lib64/ld-linux-x86-64.so.2
	logger  147982 jakob    0r  FIFO               0,14       0t0  609636 pipe
	logger  147982 jakob    1w   CHR                1,3       0t0       4 /dev/null
	logger  147982 jakob    2w   CHR                1,3       0t0       4 /dev/null
	logger  147982 jakob    3u  unix 0x00000000bc46d033       0t0  610344 type=DGRAM (CONNECTED)

Fixes #846

internal/syscallcompat/sys_common.go

@@ -60,6 +60,11 @@ func Openat(dirfd int, path string, flags int, mode uint32) (fd int, err error)
 			flags |= syscall.O_NOFOLLOW
 		}
 	}
+
+	// os/exec expects all fds to have O_CLOEXEC or it will leak fds to subprocesses.
+	// In our case, that would be logger(1), and we did leak fds to it.
+	flags |= syscall.O_CLOEXEC
+
 	fd, err = retryEINTR2(func() (int, error) {
 		return unix.Openat(dirfd, path, flags, mode)
 	})