ash: use mempcpy() in more places

ash: use mempcpy() in more places

Most changes are taken from dash.

function                                             old     new   delta
single_quote                                         127     129      +2
stack_nputstr                                         28      29      +1
path_advance                                         209     202      -7
rmescapes                                            346     308     -38
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(add/remove: 0/0 grow/shrink: 2/2 up/down: 3/-45)             Total: -42 bytes

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