gofmt, go/printer: do not insert extra linebreaks where they may break the code

gofmt, go/printer: do not insert extra linebreaks where they may break the code

Introduced a printer mode (pmode) type and corresponding
pmode values which permit easy toggling of the current
printer mode for fine-tuning of layout.

Use the printer mode to disable potential introduction of
line breaks before a closing '}' in composite literals.

Added extra test case. Applied gofmt to src and misc.

Fixes #1365.

R=r2
CC=golang-dev
https://golang.org/cl/4008041