godoc: show source code if -src flag is set in command-line mode

This also shows the source code of exported functions in server
mode (e.g. pkg/big/?m=src).

Fixes #2360.

R=rsc
CC=golang-dev
https://golang.org/cl/5254057

src/cmd/godoc/godoc.go

@@ -1041,8 +1041,8 @@ func (h *httpHandler) getPageInfo(abspath, relpath, pkgname string, mode PageInf
 		if mode&noFiltering == 0 {
 			ast.PackageExports(pkg)
 		}
-		stripFunctionBodies(pkg)
 		if mode&showSource == 0 {
+			stripFunctionBodies(pkg)
 			pdoc = doc.NewPackageDoc(pkg, path.Clean(relpath)) // no trailing '/' in importpath
 		} else {
 			past = ast.MergePackageFiles(pkg, ast.FilterUnassociatedComments)