go1.9 no longer needs "linkmode internal" on windows.

Fixes #755
Fixes #477
Fixes #631

* Fix various issues detected by megacheck

I've ran honnef.co/go/tools/cmd/megacheck and fixed a few of the
things that came up there.

* Cleanup using Gogland

Go recently introduced proposal tags to their version tags, we
are simply allowing delve to handle it appropriately.

See:
https://github.com/golang/go/commit/0954fdd51e390e209317d4652d7effc15512fd50

* dwarf/line: bugfix: not all values of the state machine can be used

According to DWARF Version 3 Section 6.2 "Line Number Information" not
all the values transversed by the line numbers state machine are valid
instructions, only the ones after a "special opcode", after the
standard opcode DW_LNS_copy and the extended opcode
DW_LINE_end_sequence.

DWARF3 describes this by specifying that only the opcodes listed above
"append a row to the matrix".

Additionally the implementation of DW_LNS_const_add_pc was wrong.

Fixes #664

* dwarf/line: fixed test failing with go1.8

* service/test: fix prologue detection tests

The conditions about which function prologue is emitted by the compiler
changed in go1.8, changed the test program so that callme2 will still
have a prologue under go1.8.

* service/test: fix step test

compilation units are linked in a different order under go1.8 so the
code of 'fmt' is no longer located after 'main' in the executable,
changed the tests so that they don't rely on this assumption anymore.

* proc: change runtime.Breakpoint support for go1.8

Before 1.8 it was sufficient to step twice to exit a
runtime.Breakpoint(), but go 1.8 added frame pointer tracking to small
functions making runtime.Breakpoint longer.
This changes runtime.Breakpoint handling in Continue to single step as
many times as are needed to exit runtime.Breakpoint.

* proc/test: fix TestIssue561 for go1.8

The go compiler changed and break statements no longer get compiled to
nothing when -N is passed:

https://go-review.googlesource.com/#/c/19848/
https://go-review.googlesource.com/#/c/19854/

fixes issue #179

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