fix: make zfs_strerror really thread-safe and portable

#15793 wanted to make zfs_strerror threadsafe, unfortunately, it
turned out that strerror_l() usage was wrong, and also, some libc
implementations dont have strerror_l().

zfs_strerror() now simply calls original strerror() and copies the
result to a thread-local buffer, then returns that.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Signed-off-by: Richard Kojedzinszky <richard@kojedz.in>
Closes #15793
Closes #16640
Closes #16923