Previously either the terminal client or the debugger service would
either lock main goroutine to a thread or provide a locked goroutine to
run _all_ DebuggedProcess functions in. This is unnecessary because only
ptrace functions need to be run from the same thread that originated the
PT_ATTACH request.

Here we use a specific thread-locked goroutine to service any ptrace
request. That goroutine is also responsible for the initial spawning /
attaching of the process, since it must be responsible for the PT_ATTACH
request.

Hardware breakpoints are by definition architecture dependant. Move them
off the DebuggedProcess struct and onto the associated arch struct.

* Set hardware breakpoints on all existing threads
* Set hardware breakpoints on any new thread the spawns

Fixes #111

* Properly find next source line for goroutines blocked in chanrecv
* Refactor breakpoint clearing
* Refactor temp breakpoint setting

* Better tracking of current goroutine
* More efficient, eliminates superfluous step syscalls
* Handles concurrency and thread coordination better

As of go version 1.4 the standard library syscall package is "locked
down" and code outside of the standard library is recommended to migrate
to the go.sys subrepository.

Reference: https://golang.org/s/go1.4-syscall

Enable usage of dr1-dr3.  Clear control bits when a breakpoint
is disabled. Use DR_LEN_1 instead of DR_LEN_8 so breakpoint work on
unaligned adresses.

Fixes #51.

Improves error message on systems that delve is failing on.

Currently only works for amd64 processors.