ash: add support for bash 'function' keyword

Where the POSIX shell allows functions to be defined as:

   name () compound-command [ redirections ]

bash adds the alternative syntax:

   function name [()] compound-command [ redirections ]

Implement this in ash's bash compatibility mode.  Most compound
commands work (for/while/until/if/case/[[]]/{}); one exception is:

   function f (echo "no way!")

The other two variants work:

   f() (echo "ok")
   function f() (echo "also ok")

function                                             old     new   delta
parse_command                                       1555    1744    +189
tokname_array                                        232     240      +8
.rodata                                           155612  155566     -46
------------------------------------------------------------------------------
(add/remove: 0/0 grow/shrink: 2/1 up/down: 197/-46)           Total: 151 bytes

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

shell/ash_test/ash-misc/func_bash1.tests

@@ -0,0 +1,28 @@
+function f() { echo $1; }
+f 1
+
+function f() ( echo $1; )
+f 2
+
+function f() ( echo $1 )
+f 3
+
+function f() for i in 1 2 3; do
+	echo $i
+done
+f
+
+function f { echo $1; }
+f 1
+
+# the next two don't work
+#function f ( echo $1; )
+f 2
+
+#function f ( echo $1 )
+f 3
+
+function f for i in 1 2 3; do
+	echo $i
+done
+f