swapon/swapoff: size reduction, cleanup, fixes, improvements

1) real swapon/swapoff handles also devices on the commandline with -a;
2) xstat(device)  in  swap_enable_disable aborts on error when cycling through
   fstab so some devices  are not handled;
3) duplicated code for ENABLE_FEATURE_SWAPON_DISCARD and
   ENABLE_FEATURE_SWAPON_PRI was moved to functions.
4) silence some error messages with -a;
5) minor cleanups and code refactoring reduced the size as per bloat-check:
6) I also added support for /proc/swaps handling to swapoff:
"When the -a flag is given, swapping is disabled on all known  swap  devices
 and  files  (as  found  in  /proc/swaps  or /etc/fstab)."
So now swapoff first cycles through  /proc/swaps and then through fstab
to swapoff all devices.

function                                             old     new   delta
set_discard_flag                                       -     106    +106
swap_enable_disable                                  147     238     +91
set_priority_flag                                      -      79     +79
retrieve_file_data                                   470     467      -3
swap_on_off_main                                     638     418    -220
------------------------------------------------------------------------------
(add/remove: 2/0 grow/shrink: 1/2 up/down: 276/-223)           Total: 53 bytes

Signed-off-by: Tito Ragusa <farmatito@tiscali.it>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>
swapon/swapoff: -a returns 0 on ignored errors

Signed-off-by: Tito Ragusa <farmatito@tiscali.it>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>
build system: specify '-nostldlib' when linking to .o files

For certain cross build scenarios the LD variable is overridden
to use the gcc executive to ensure all the target tuning parameters
are used.  In these cases, the executive errors out as shown below
but since this step is only linking to a .o file the standard libs
are not needed.

    $ make LD=gcc applets/built-in.o
      LD      applets/built-in.o
    /usr/bin/ld: cannot find -lgcc_s
    /usr/bin/ld: cannot find -lgcc_s
    collect2: ld returned 1 exit status
    make[1]: *** [applets/built-in.o] Error 1
    make: *** [applets_dir] Error 2

Signed-off-by: Drew Moseley <drew_moseley@mentor.com>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>
vi: undo support for vi with intermediate queuing

function                                             old     new   delta
undo_push                                              -     411    +411
undo_pop                                               -     288    +288
do_cmd                                              4160    4426    +266
char_insert                                          363     483    +120
undo_queue_commit                                      -      61     +61
text_hole_delete                                     108     163     +55
string_insert                                         94     127     +33
colon                                               2864    2882     +18
yank_delete                                           92     101      +9
vi_main                                              273     280      +7
dot_scroll                                            88      93      +5
dot_right                                             29      34      +5
dot_prev                                              20      25      +5
dot_next                                              20      25      +5
dot_left                                              24      29      +5
dot_end                                               20      25      +5
dot_begin                                             20      25      +5
init_text_buffer                                     154     156      +2
text_hole_make                                       145     142      -3
file_insert                                          333     318     -15
------------------------------------------------------------------------------
(add/remove: 3/0 grow/shrink: 15/2 up/down: 1305/-18)        Total: 1287 bytes

(without queuing it's ~870 bytes)

Signed-off-by: Jody Bruchon <jody@jodybruchon.com>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>
vi: undo code shrink

function                                             old     new   delta
undo_push                                            414     395     -19
do_cmd                                              4803    4761     -42

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>
vi: clear undo buffer when we change to another file

function                                             old     new   delta
init_text_buffer                                     156     190     +34
undo_push                                            360     382     +22
count_lines                                           74      72      -2
undo_pop                                             246     222     -24
------------------------------------------------------------------------------
(add/remove: 0/0 grow/shrink: 2/2 up/down: 56/-26)             Total: 30 bytes

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>
Merge branch 'master' of git://git.busybox.net/busybox into kitkat