hush: use ash's ulimit builtin; make it more more bash0like while at it

Based on a patch by Tobias Klauser <tklauser@distanz.ch>

function                                             old     new   delta
shell_builtin_ulimit                                   -     498    +498
limits_tbl                                            33      88     +55
ulimit_opt_string                                      -      38     +38
bltins1                                              288     300     +12
limits_name                                          127       -    -127
ulimitcmd                                            415       7    -408
------------------------------------------------------------------------------
(add/remove: 3/1 grow/shrink: 2/1 up/down: 603/-535)           Total: 68 bytes
   text    data     bss     dec     hex filename
 839229     453    6828  846510   ceaae busybox_old
 839423     453    6828  846704   ceb70 busybox_unstripped

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>
diff --git a/shell/Kbuild b/shell/Kbuild
index d8306dc..8b52865 100644
--- a/shell/Kbuild
+++ b/shell/Kbuild
@@ -5,8 +5,8 @@
 # Licensed under the GPL v2, see the file LICENSE in this tarball.
 
 lib-y:=
-lib-$(CONFIG_ASH)      += ash.o ash_ptr_hack.o shell_common.o builtin_read.o
-lib-$(CONFIG_HUSH)     += hush.o match.o shell_common.o builtin_read.o
+lib-$(CONFIG_ASH)      += ash.o ash_ptr_hack.o shell_common.o builtin_read.o builtin_ulimit.o
+lib-$(CONFIG_HUSH)     += hush.o match.o shell_common.o builtin_read.o builtin_ulimit.o
 lib-$(CONFIG_CTTYHACK) += cttyhack.o
 
 lib-$(CONFIG_SH_MATH_SUPPORT) += math.o