lint: add new linter configuration

We use the golangci-lint in lnd and it has quite a few more detections
enabled than what's in the current gotest.sh script.
We don't start with a given base commit on purpose but instead fix
everything the linter finds in the following commits.

.golangci.yml

@@ -0,0 +1,54 @@
+run:
+  # timeout for analysis
+  deadline: 10m
+
+linters-settings:
+  govet:
+    # Don't report about shadowed variables
+    check-shadowing: false
+  gofmt:
+    # simplify code: gofmt with `-s` option, true by default
+    simplify: true
+
+linters:
+  enable-all: true
+  disable:
+    # Global variables are used in many places throughout the code base.
+    - gochecknoglobals
+
+    # Some lines are over 80 characters on purpose and we don't want to make them
+    # even longer by marking them as 'nolint'.
+    - lll
+
+    # We don't care (enough) about misaligned structs to lint that.
+    - maligned
+
+    # We have long functions, especially in tests. Moving or renaming those would
+    # trigger funlen problems that we may not want to solve at that time.
+    - funlen
+
+    # Disable for now as we haven't yet tuned the sensitivity to our codebase
+    # yet.  Enabling by default for example, would also force new contributors to
+    # potentially extensively refactor code, when they want to smaller change to
+    # land.
+    - gocyclo
+
+    # Instances of table driven tests that don't pre-allocate shouldn't trigger
+    # the linter.
+    - prealloc
+
+    # Init functions are used by loggers throughout the codebase.
+    - gochecknoinits
+    
+    # Explicit types are okay.
+    - interfacer
+
+issues:
+  exclude-rules:
+    # Exclude gosec from running for tests so that tests with weak randomness
+    # (math/rand) will pass the linter.
+    - path: _test\.go
+      linters:
+        - gosec
+        - errcheck
+        - dupl