hush: fix "getopts" builtin to not be upset by other builtins calling getopt()

function                                             old     new   delta
builtin_getopts                                      363     403     +40
unset_local_var_len                                  185     215     +30
set_local_var                                        440     466     +26
reset_traps_to_defaults                              151     157      +6
pseudo_exec_argv                                     320     326      +6
install_special_sighandlers                           52      58      +6
pick_sighandler                                       62      65      +3
execvp_or_die                                         85      88      +3
------------------------------------------------------------------------------
(add/remove: 0/0 grow/shrink: 8/0 up/down: 120/0)             Total: 120 bytes

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

shell/ash_test/ash-getopts/getopt_nested.right

@@ -0,0 +1,14 @@
+var:a
+var:b
+var:c
+var:a
+var:b
+var:c
+Illegal option -d
+var:?
+Illegal option -e
+var:?
+Illegal option -f
+var:?
+var:a
+End: var:? OPTIND:6

shell/ash_test/ash-getopts/getopt_nested.tests

@@ -0,0 +1,21 @@
+# Test that there is no interference of getopt()
+# in getopts and unset.
+# It's unclear what "correct" OPTIND values should be
+# for "b" and "c" results from "-bc": 2? 3?
+# What we focus on here is that all options are reported
+# correct number of times and in correct sequence.
+
+(
+
+loop=99
+while getopts "abc" var -a -bc -abc -def -a; do
+    echo "var:$var" #OPTIND:$OPTIND
+    # this may use getopt():
+    unset -ff func
+    test $((--loop)) = 0 && break  # BUG if this triggers
+done
+echo "End: var:$var OPTIND:$OPTIND"
+
+) 2>&1 \
+| sed   -e 's/ unrecognized option: / invalid option -- /' \
+        -e 's/ illegal option -- / invalid option -- /' \