Fixes a code path where stacktrace returns < 2 locations and
thread.ReturnAddress would panic. Now returns an error.

This fix helps avoid infinite recursion.

Breakpoints, tracepoints, etc.. take a location spec as input. This
patch improves the expressiveness of that API. It allows:

* Breakpoint at line
* Breakpoint at function (handling package / receiver smoothing)
* Breakpoint at address
* Breakpoint at file:line
* Setting breakpoint based off regexp

Fixes #192

fixes issue #179

the entry point of a function is the beginning of the prologue, which can be run multiple times for each invocation of a function if the stack needs to be expanded or the scheduler needs to be run.

The process will already be killed via withTestProcess

proc.(*Thread).GetG: reading TLS memory directly for g address instead of modifying the executable code

bugfix, Issue #163: offset of g struct in TLS picked based on the value of runtime.buildVersion and presence of compile units created by GNU AS, instead of being fixed to -16

Allows a user to execute `dlv trace [regexp]` and Delve will execute the
program and output information on functions matching [regexp].