ash: remove one redundant function, improve string sharing, better field names

ash: remove one redundant function, improve string sharing, better field names

function                                             old     new   delta
var_end                                                -      15     +15
changepath                                           194     192      -2
setvareq                                             222     218      -4
lookupvar                                             57      51      -6
bltinlookup                                           57      51      -6
evalfun                                              452     443      -9
varequal                                              14       -     -14
------------------------------------------------------------------------------
(add/remove: 1/1 grow/shrink: 0/5 up/down: 15/-41)            Total: -26 bytes

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>