ash: perform tilde expansion in all parameter expansion words

Previously tilde expansion was not carried out for =?#% expansion words.

Test case:
   a=~root:~root
   echo ${a#~root}
Old result:
   /root:/root
New result:
   :/root

Based on commit dd721f71 from git://git.kernel.org/pub/scm/utils/dash/dash.git
by Herbert Xu

function                                             old     new   delta
subevalvar                                          1152    1153      +1

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

shell/ash_test/ash-vars/var-expand-tilde-in-parameter-expansion.tests

@@ -0,0 +1,2 @@
+a=~root:~root
+echo ${a#~root}