This change adds support for logging JSON records when the --log.json flag is
given. The --debug and --backtrace flags are deprecated and replaced by
--log.debug and --log.backtrace.

While changing this, it was noticed that the --memprofilerate and
--blockprofilerate were ineffective (they were always overridden even if
--pprof.memprofilerate was not set). This is also fixed.
Co-authored-by: Felix Lange <fjl@twurst.com>