Devtabs initialize all methods

It looks like the style for struct dev is to fill in the entries, using
devwhatever() for ones the specific device doesn't provide.  This patch does so
for the hacked on power and config methods, which were implicitly 0 before.

I'll need this so I can add another op that will come after power and config.

The alternative is to initialize each member explicitly (.init = devinit, etc).
Its more of a question of how you handle your entries that the device does
nothing with: either 0 or a default.  If we went with 0, then the explicit
initialization is the way to go.  Since the style seems to be default
initializers, then we should just make sure everything has a default method.
Adds chaninfo method to struct dev

With a default of devchaninfo().  These functions should return a useful string
describing chan.  They can use the ret buffer, passed in by the caller, in case
they want to snprintf or something.
Adds ipchaninfo

Parses the basics of the qid for a file, based off ipwrite().  Feel free to
extend it to handle other things (like protodirs, etc).
Fixes plan9.h include ifdefs

Need to #define something, so we don't include plan9.h's contents multiple
times.
Procinfo prints out info about plan9 files