The Phase 1 syslog migration used POSIX syslog() on all Unix platforms per the
migration plan's simplicity recommendation. On macOS this broke Logging.Tests:
POSIX syslog() does not set the os_log subsystem/category, so 'log show' entries
never match the test's "category -eq $logId" filter and no items are returned.

Restore the native (libpsl-native nativesyslog.cpp) behavior on macOS by logging
through the unified logging system:

- os_log_create("com.microsoft.powershell", ident) sets subsystem + category.
- _os_log_impl(...) is invoked with a hand-built argument buffer equivalent to
  what the os_log compiler macro emits for a single "%{public}s" argument.
- Severity is mapped to os_log_type_t (fault/error/debug/default) matching the
  native switch. Linux continues to use POSIX syslog().

NOTE: os_log is normally a compiler-macro API; the _os_log_impl + DSO/format-string
path is compile-verified only (host is Windows). macOS CI (Logging.Tests.ps1) must
confirm 'log show' renders the eventMessage.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

…uture phase

POSIX syslog() is not a valid substitute for os_log on macOS: Host/Logging.Tests.ps1
reads entries via 'log show --style json' and filters on the os_log category (= logId)
and messageType "Default". syslog() sets no category, so no entries match.

A pure-managed os_log (os_log_create + hand-rolled _os_log_impl) compiles and sets the
category correctly, but 'log show' renders a compose failure because os_log requires the
format string to be a compile-time constant located in a Mach-O __TEXT,__os_log section;
a runtime/heap format string is not resolvable by logd. This is why mature FFI bindings
ship a small C shim.

Interim resolution:
- Linux: managed libc syslog (unchanged).
- macOS: route OpenLog/SysLog/CloseLog back to libpsl-native
  Native_OpenLog/Native_SysLog/Native_CloseLog.

A dedicated macOS os_log shim (compiled during the macOS build with a literal
"%{public}s" format) is documented as a future migration phase; it will let the
libpsl-native logging dependency be removed on macOS as well.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

…Phase 3)

Replace the libpsl-native ForkAndExecProcess P/Invoke used by SSH based
remoting with a pure-managed implementation over libc posix_spawn(3).

- Add engine/Interop/Unix/Spawn.cs: Interop.Unix.SpawnProcess sets up
  close-on-exec pipes for the requested stdin/stdout/stderr redirections,
  builds posix_spawn file actions (adddup2, addchdir_np) and attributes,
  and spawns the child. posix_spawn is used instead of a hand-rolled
  fork/execve because running managed code between fork and exec is unsafe.
- Preserve SUPPRESS_PROCESS_SIGINT semantics via POSIX_SPAWN_SETSID so the
  SSH child runs in its own session and terminal Ctrl+C (SIGINT) does not
  propagate to it (flag value differs per OS: Linux 0x0080, macOS 0x0400).
- RunspaceConnectionInfo.StartSSHProcess now calls Interop.Unix.SpawnProcess;
  remove the now-dead CreateProcess, ForkAndExecProcess DllImport,
  AllocNullTerminatedArray, FreeArray and the SUPPRESS_PROCESS_SIGINT const.

This removes the last real [DllImport("libpsl-native")] P/Invoke; only the
macOS os_log syslog path remains (deferred to a future phase).

Compile-verified on Windows and cross-compiled linux-x64. Not runtime-tested;
SSH remoting startup, stream redirection, and Ctrl+C isolation should be
validated on Linux and macOS.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>