all: fix or silence various gcc warnings As usual, gcc finds some real problems but also reports a ton of noise. Fix the problems and quiet the noise.

svgpic: new program to convert pic to svg This is an experiment. Like tpic it's a copy-and-paste fork of pic. Change-Id: Ia22772bd5881c7904a6d8f8e0b46fde8cea89cbd Reviewed-on: https://plan9port-review.googlesource.com/2920 Reviewed-by: Russ Cox <rsc@swtch.com>