6a/a.h:
. Dropped <u.h> and <libc.h>.
. Made definition of EOF conditional.

6a/a.y:
. Added <u.h> and <libc.h>.

6a/lex.c:
. Added <u.h> and <libc.h>.
. Dropped <ctype.h> (now in <u.h>).

6c/gc.h:
. Added varargck pragma for "lD".

6c/swt.c:
. Dropped unused "thestring" argument in Bprint() calls.

6l/Makefile:
. Dropped unneeded directory prefix.

6l/l.h:
. Dropped unneeded directory prefix.
. Added varargck pragma for "I" and "i".

6l/obj.c:
. Dropped unneeded assignment.
. Dropped unreachable goto statement.

6l/pass.c:
. Dropped assignments flagged as unused.

6l/prof.c:
. Replaced "#if 0" with "#ifdef NOTDEF".

6l/span.c:
. Dropped unused incrementation.
. Added USED() as required.
. Dropped unreachable "return" statement.

R=golang-dev
CC=golang-dev, rsc
https://golang.org/cl/4747044