cmd/dist: add -Wstrict-prototypes to CFLAGS and fix all the compiler errors
Plan 9 compilers insist this but as we don't have Plan 9
builders, we'd better let gcc check the prototypes.

Inspired by CL 7289050.

R=golang-dev, seed, dave, rsc, lucio.dere
CC=akumar, golang-dev
https://codereview.appspot.com/7288056