fix: posix_spawn returns errno on error, not -1

[SteveLauC]

What does this PR do

According to the POSIX man page:

Otherwise, no child process shall be created, the value stored into the variable pointed to by a non-NULL pid is unspecified, and an error number shall be returned as the function return value to indicate the error.

Error::result() assumes the function returns -1 on error, so it won't work here.

Actually, this is my fault 🤦‍♂️, the implementation in PR #2007 handled this correctly, but my erroneous review comment led the PR in the wrong direction.

Also, since posix_spawn() uses fork() (can be other syscalls depending on the impl) under the hood, its tests should grab the FORK_MTX lock.

Checklist:

• I have read CONTRIBUTING.md
• I have written necessary tests and rustdoc comments
• A change log has been added if this PR modifies nix's API

These 2 posx_spawn*() interfaces have not been released, so no changelog needed.