feat(commands) add cast error
explain(commands/http)

we've gotta allow the code to speak for itself. I wouldn't have been
able to safely modify this code without having my hand held. I am but a
lowly programmer with a simple mind.

cc @jbenet @whyrusleeping @mappum
refactor(commands/optionvalue) use

@mappum this optionvalue is a really great abstraction. It makes
maintenance super simple!

@jbenet @whyrusleeping
tests(commands/option) test the OptionValue methods

TODO add tests for remaning, untested methods.
fix(commands/optionvalue) don't shadow the return variable

NB: return variables are provided for clarity. Otherwise, it's not
entirely clear that the bool is for _found_.
fix(commands/http/handler) check if found

@mappum, could you CR this and let me know if I've interpreted the
desired behavior correctly?
fix(commands/mount_unix, optionvalue) handle found, !found

@mappum, could you provide a quick LGTM?
fix(mount_unix) optionvalue signature

@mappum can you just sanity check me here with a quick LGTM CR?
fix(2/pin) optionvalue signature
fix(2/refs) optionvalue signature

@mappum this one is straightforward, but tagging you here just in case
fix(2/version) option value sig
fix(2/init, 2/add) fix optionvalue signature
fix(2/main) option value signature
refactor(mount_unix) return the errs in select

fix(mount_unix) rm extraneous error
rename variable
question(mount_unix) add todo regarding mount timeout

@whyrusleeping @jbenet

is this non-deterministic?
fix(commands/optionvalue) use the util.ErrCast()