proc: switch /proc/cmdline to seq_file

and move it to fs/proc/cmdline.c while I'm at it.

Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>

fs/proc/Makefile

@@ -9,6 +9,7 @@ proc-$(CONFIG_MMU)	:= mmu.o task_mmu.o
 
 proc-y       += inode.o root.o base.o generic.o array.o \
 		proc_tty.o proc_misc.o
+proc-y	+= cmdline.o
 proc-y	+= loadavg.o
 proc-y	+= meminfo.o
 proc-y	+= uptime.o