hush: rework input char buffering to allow more than one-deep peek

This fixes backslash+newline continuation in
	$VAR\
	NAME
construct. (ash has a bug there as well).

function                                             old     new   delta
file_peek2                                             -      74     +74
parse_dollar                                         746     773     +27
expand_vars_to_list                                 1143    1167     +24
setup_string_in_str                                   32      46     +14
setup_file_in_str                                     33      47     +14
file_get                                             264     278     +14
static_peek2                                           -       7      +7
file_peek                                             91      72     -19
------------------------------------------------------------------------------
(add/remove: 2/0 grow/shrink: 5/1 up/down: 174/-19)           Total: 155 bytes

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

shell/ash_test/ash-vars/var_subst_in_for.tests

@@ -0,0 +1,40 @@
+if test $# = 0; then
+    exec "$THIS_SH" "$0" abc "d e"
+fi
+
+echo 'Testing: in x y z'
+for a in x y z; do echo ".$a."; done
+
+echo 'Testing: in u $empty v'
+empty=''
+for a in u $empty v; do echo ".$a."; done
+
+echo 'Testing: in u " $empty" v'
+empty=''
+for a in u " $empty" v; do echo ".$a."; done
+
+echo 'Testing: in u $empty $empty$a v'
+a='a'
+for a in u $empty $empty$a v; do echo ".$a."; done
+
+echo 'Testing: in $a_b'
+a_b='a b'
+for a in $a_b; do echo ".$a."; done
+
+echo 'Testing: in $*'
+for a in $*; do echo ".$a."; done
+
+echo 'Testing: in $@'
+for a in $@; do echo ".$a."; done
+
+echo 'Testing: in -$*-'
+for a in -$*-; do echo ".$a."; done
+
+echo 'Testing: in -$@-'
+for a in -$@-; do echo ".$a."; done
+
+echo 'Testing: in $a_b -$a_b-'
+a_b='a b'
+for a in $a_b -$a_b-; do echo ".$a."; done
+
+echo Finished