unzip: make options parsing more robust on getopt w/o gnu extensions

unzip: make options parsing more robust on getopt w/o gnu extensions

Also, code shrank:

function                                             old     new   delta
static.extn                                           15      10      -5
packed_usage                                       29231   29217     -14
unzip_main                                          2388    2291     -97
------------------------------------------------------------------------------
(add/remove: 0/0 grow/shrink: 0/3 up/down: 0/-116)           Total: -116 bytes

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