These changes are not particularly invasive and have been tested
as broadly as possible.

8l/l.h:
  -	#pragma varargck: added some, removed duplicates.

ld/dwarf.c:
  -	As Plan 9 has no NULL, changed all occurrences to nil.
  -	Added USED(size); where necessary.
  -	Added (void) argument in definition of finddebugruntimepath().
  -	Plan 9 compiler was complaining about multiple
        assignments, repeaired by breaking up the commands.
  -	Correction: havedynamic = 1; restored.

ld/go.c:
  -	Needed USED(file); in two functions.
  -	Removed unused assignments flagged by the Plan 9 compiler.

ld/lib.c:
  -	Replaced unlink() with remove() which seems available everywhere.
  -	Removed USED(c4); and USED(magic) no longer required.
  -	Removed code flagged as unused by the Plan 9 compiler.
  -	Added attributes to a number of format strings.

R=rsc
CC=golang-dev
https://golang.org/cl/4435047