Copy stable/9 to releng/9.3 as part of the 9.3-RELEASE cycle.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

MFC 250421: posix_spawn_file_actions_adddup2(3): Document difference with
dup2().

The ability to clear a file descriptor's close-on-exec flag via
posix_spawn_file_actions_adddup2() is in fact proposed in Austin Group issue
#411.

MFC r250412: posix_spawn_file_actions_addopen(3): Correct error for bad file
descriptor.

As per POSIX.1-2008, posix_spawn_file_actions_add* return [EBADF] if a file
descriptor is negative, not [EINVAL]. The bug was only in the manual page;
the code is correct.

Copy head to stable/9 as part of 9.0-RELEASE release cycle.

Approved by: re (implicit)

Use our canonical .Dd format.

Submitted by: Ulrich Spoerlein

Fix the email address formats in some posix_spawn(3) manpages.

It seems I made a small bug when writing some of the posix_spawn(3)
manpages. Remove the redundant "Ed Schouten", which broke the AUTHORS
section.

Approved by: philip (mentor, implicit)

Add manual pages for posix_spawn() functions.

PR: standards/122051