Fix FD leaks and add missing CLOEXEC flags

[xroche]

What does this PR do?

Fix file descriptor leaks on error paths and add missing CLOEXEC flags across several files. In library mode, ddprof_start_profiling() fork+execs the daemon — every FD without CLOEXEC in the parent service leaks into the child process.

Changes

File	Fix
src/create_elf.cc	Use UniqueFd for RAII cleanup (fixes FD leak on error paths), add O_CLOEXEC, fix "prox" typo
src/ringbuffer_utils.cc	Close mapfd on ftruncate/eventfd error paths, add EFD_CLOEXEC to eventfd()
src/ipc.cc	Use accept4() with SOCK_CLOEXEC (parent socket uses it but accepted connections didn't inherit it), add SFD_CLOEXEC to signalfd()
src/daemonize.cc	Add O_CLOEXEC to pipe2()
src/ddprof_module_lib.cc	Add O_CLOEXEC to open() calls (already uses UniqueFd)

Motivation

For long-running services using ddprof in library mode (e.g., Algolia's 7 production services), leaked FDs accumulate across profiler restarts. The missing CLOEXEC flags cause FDs to leak into the fork+exec'd daemon child on every profiler start.

How to test the change?

• create_elf.cc: trigger an error path (e.g., corrupt ELF) and verify no FD leak with /proc/self/fd count
• ringbuffer_utils.cc: trigger ftruncate failure (e.g., ENOSPC on memfd) and verify mapfd is closed
• ipc.cc: verify accepted connections have CLOEXEC set via /proc/<pid>/fdinfo/<fd>
• daemonize.cc, ddprof_module_lib.cc: verify CLOEXEC flag on opened FDs

Classification

• CWE-775: Missing Release of File Descriptor or Handle after Effective Lifetime
• CWE-403: Exposure of File Descriptor to Unintended Control Sphere

Fixes #497