libzpool/cmn_err: remove suppression, add stop option, cleanup

A small uplift of the cmn_err() and panic() calls in userspace:

• remove the suppression on CE_NOTE. We have very few of these calls in a standard build, it's convenient for "print debugging".

• make prefixes clear and consistent.

• add LIBZPOOL_PANIC_STOP environment variable to send SIGSTOP to the process group on a panic, rather than abort(), so all threads remain alive for inspection.

Sponsored-by: Klara, Inc.
Sponsored-by: Wasabi Technology, Inc.
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Alexander Motin <alexander.motin@TrueNAS.com>
Signed-off-by: Rob Norris <rob.norris@klarasystems.com>
Closes #17834