ash: do not expand tilde in parameter expansion within quotes

ash: do not expand tilde in parameter expansion within quotes

Test case:
   unset a
   echo "${a:-~root}"
Old result:
   /root
New result:
   ~root

Based on commit 170f44d from git://git.kernel.org/pub/scm/utils/dash/dash.git
by Herbert Xu

function                                             old     new   delta
evalvar                                              598     604      +6
parse_command                                       1440    1443      +3
localcmd                                             325     327      +2
readtoken1                                          3199    3200      +1
argstr                                              1180    1164     -16
------------------------------------------------------------------------------
(add/remove: 0/0 grow/shrink: 4/1 up/down: 12/-16)             Total: -4 bytes

Signed-off-by: Ron Yorston <rmy@pobox.com>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>