Fix for xeth OOM issue

There are a bunch of changes required to make this work:

- in miner: allow unregistering agents, fix RemoteAgent.Stop
- in eth/filters: make FilterSystem.Stop not crash
- in rpc/comms: move listen loop to platform-independent code

Fixes #1930. I ran the shell loop there for a few minutes and didn't see
any changes in the memory profile.

Argument parsing can lead to panic in rpc channel

eth: don't block sync goroutines that short circuit

 check argument type before parsing params
 recover from panic in ipc channel

cmd/utils, crypto: add --lightkdf flag for lighter KDF

makefile: fix evm ld flags, pass them to xgo too

rpc api: eth_getNatSpec

eth, xeth: fix GasPriceOracle goroutine leak

XEth.gpo was being initialized as needed. WithState copies the XEth
struct including the gpo field. If gpo was nil at the time of the copy
and Call or Transact were invoked on it, an additional GPO listenLoop
would be spawned.

Move the lazy initialization to GasPriceOracle instead so the same GPO
instance is shared among all created XEths.

Fixes #1317
Might help with #1930

* xeth, rpc: implement eth_getNatSpec for tx confirmations
* rename silly docserver -> httpclient
* eth/backend: httpclient now accessible via eth.Ethereum init-d via config.DocRoot
* cmd: introduce separate CLI flag for DocRoot (defaults to homedir)
* common/path: delete unused assetpath func, separate HomeDir func

tests: update JSON files, add new wrappers

eth: time out status message exchange after 5s

fix console history, lines with leadning whitespace NOT included

core: fix #1921, decode all receipt field, not just consensus

console, cli, api fixes

* lines with leading space are ommitted from history
* exit processed even with whitespace around
* all whitespace lines (not only empty ones) are ignored

add 7 missing commands to admin api autocomplete

registrar: methods now return proper error if reg addresses are not set. fixes #1457

rpc/console: fix personal.newAccount() regression. Now all comms accept interactive password

registrar: add registrar tests for errors

crypto: catch AES decryption error on presale wallet import + fix error msg format. fixes #1580

CLI: improve error message when starting a second instance of geth. fixes #1564

cli/accounts: unlock multiple accounts. fixes #1785
* make unlocking multiple accounts work with inline <() fd
* passwdfile now correctly read only once
* improve logs
* fix CLI help text for unlocking

fix regression with docRoot / admin API
* docRoot/jspath passed to rpc/api ParseApis, which passes onto adminApi
* docRoot field for JS console in order to pass when RPC is (re)started
* improve flag desc for jspath

common/docserver: catch http errors from response

fix rpc/api tests

common/natspec: fix end to end test (skipped because takes 8s)

registrar: fix major regression:
* deploy registrars on frontier
* register HashsReg and UrlHint in GlobalRegistrar.
* set all 3 contract addresses in code
* zero out addresses first in tests

core/vm: JIT segmentation

eth/63 fast synchronization algorithm

core, tests: get_hash fix

Make sure that we're fetching the hash from the current chain and not
the canonical chain.

core, core/state: move gas tracking out of core/state