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

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>