fix various typos in comments, docs and log messages.

Use dev/apm/files.apm for APM options in opt_apm.h.

Now both apm and apmdev files.apm can be included so you can switch
from apmdev0 to apm0 at hpcapm0 in your kernel config easily.

Config glue for attaching apm at apmdevif.

Unfortunately dev/apm/files.apm and dev/hpc/apm/files.apm cannot be
included both at the same time (they define same options and config(1)
complains), so y

Config glue for attaching apm at apmdevif.

Unfortunately dev/apm/files.apm and dev/hpc/apm/files.apm cannot be
included both at the same time (they define same options and config(1)
complains), so you need to comment out old code and uncomment new code
to be able to replace "apmdev0 at hpcapm0" with "apm0 at hpcapm0" in
your kernel config.

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Remove (the default) #define XXXDEBUG and add the options to the relevant
files.xxx file.

Rework hpcapm(4) attachment definition so it is not required to attach at
mainbus, and convert its two users, hpcsh and hpcmips, to that.

Many thanks to Valeriy Ushakov and Ben Collver for testing.