gc: clean up printing.

gc: clean up printing.

Got rid of all the magic mystery globals. Now
for %N, %T, and %S, the flags +,- and # set a sticky
debug, sym and export mode, only visible in the new fmt.c.
Default is error mode. Handle h and l flags consistently with
the least side effects, so we can now change
things without worrying about unrelated things
breaking.

fixes #2361

R=rsc
CC=golang-dev
https://golang.org/cl/5316043