[core] use posix_spawn() where available

use posix_spawn() where available
use posix_spawn_file_actions_addfchdir_np() where available

Using posix_spawn() reduces initial execution overhead of CGI programs;
posix_spawn() is often faster than application code wrapping and calling
traditional fork(),execve().

(history: fdevent.c posix_spawn code based on fdio.c:fdio_ipc_spawn()
 from 2015 on one of my unpublished branches.  The inability to chdir()
 delayed inclusion in lighttpd, as the CGI specification says:
 "The current working directory for the script SHOULD be set
  to the directory containing the script."
 e.g. chdir() to target program directory before CGI execution)

posix_spawn_file_actions_addfchdir_np() is a new(er) extension supported
in glibc 2.29+, musl libc, FreeBSD ≥ 13.1, macOS ≥ 10.15 according to
https://cygwin.com/pipermail/cygwin/2023-April/253505.html
https://cygwin.com/pipermail/cygwin/2023-April/253526.html
https://sourceware.org/bugzilla/show_bug.cgi?id=17405

POSIX Issue 8 plans to include posix_spawn_file_actions_addfchdir():
https://www.austingroupbugs.net/view.php?id=1208

1 files changed, 2 insertions, 0 deletions

diff --git a/configure.ac b/configure.ac
index 02b1cdf6..331f95d2 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1461,6 +1461,8 @@ AC_CHECK_FUNCS([\
   mkostemp \
   pipe2 \
   poll \
+  posix_spawn \
+  posix_spawn_file_actions_addfchdir_np \
   pread \
   pwrite \
   sendfile \