hush: fix ${##}, ${#?}, ${#!} handling

function                                             old     new   delta
parse_dollar                                         786     820     +34
expand_one_var                                      1579    1592     +13
------------------------------------------------------------------------------
(add/remove: 0/0 grow/shrink: 2/0 up/down: 47/0)               Total: 47 bytes

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

shell/ash_test/ash-vars/param_expand_alt.tests

@@ -6,7 +6,7 @@
 # now some funky ones.
 # ${V+word} "if V unset, then substitute nothing, else substitute word"
 # ${V:+word} "if V unset or '', then substitute nothing, else substitute word"
-# bash doesn't accept ${#+}. ash prints 0 (not $#).
+# bash doesn't accept ${#+}. ash prints 0 (not $#): "len of $+"
 echo _${#+}_ _${#:+}_
 # Forms with non-empty word work as expected in both ash and bash.
 echo _${#+z}_ _${#:+z}_

shell/ash_test/ash-vars/param_expand_len1.right

@@ -0,0 +1,11 @@
+One:1
+Two:2
+Three:3
+
+One:1
+Two:2
+Three:3
+
+Ok ${#$}: 0
+
+Ok ${#!}: 0

shell/ash_test/ash-vars/param_expand_len1.tests

@@ -0,0 +1,31 @@
+# ${#c} for any single char c means "length of $c", including all special vars
+
+false
+echo One:${#?}
+(exit 10)
+echo Two:${#?}
+(exit 100)
+echo Three:${#?}
+
+echo
+echo One:${##}
+set -- 1 2 3 4 5 6 7 8 9 0
+echo Two:${##}
+set -- 1 2 3 4 5 6 7 8 9 0  1 2 3 4 5 6 7 8 9 0  1 2 3 4 5 6 7 8 9 0 \
+       1 2 3 4 5 6 7 8 9 0  1 2 3 4 5 6 7 8 9 0  1 2 3 4 5 6 7 8 9 0 \
+       1 2 3 4 5 6 7 8 9 0  1 2 3 4 5 6 7 8 9 0  1 2 3 4 5 6 7 8 9 0 \
+       1 2 3 4 5 6 7 8 9 0  1 2 3 4 5 6 7 8 9 0  1 2 3 4 5 6 7 8 9 0
+echo Three:${##}
+
+echo
+v=$$
+test "${#v}" = "${#$}"
+echo 'Ok ${#$}:' $?
+
+echo
+sleep 0 &
+v=$!
+test "${#v}" = "${#!}"
+echo 'Ok ${#!}:' $?
+
+# TODO: ${#-} ${#_}