tccgen/win32: let __declspec(dllimport) imply extern

Also, retain storage qualifiers in type_decl, in particular
also for function pointers.  This allows to get rid of this
very early hack in decl()
    type.t |= (btype.t & VT_STATIC); /* Retain "static". */
which was to fix the case of
    int main() { static int (*foo)(); ...

Also:
- missing __declspec(dllimport) is an error now
- except if the symbol is "_imp__symbol"
- demonstrate export/import of data in the dll example (while
  'extern' isn't strictly required with dllimport anymore)
- new function 'patch_storage()' replaces 'weaken_symbol()'
  and 'apply_visibility()'
- new function 'update_storage()' applies storage attributes
  to Elf symbols.
- put_extern_sym/2 accepts new pseudo section SECTION_COMMON
- add -Wl,-export-all-symbols as alias for -rdynamic
- add -Wl,-subsystem=windows for mingw compatibility
- redefinition of 'sym' error for initialized global data

libtcc.c

@@ -1299,6 +1299,8 @@ static int link_option(const char *str, const char *val, const char **ptr)
         if (*p != ',' && *p != '=')
             return 0;
         p++;
+    } else if (*p) {
+        return 0;
     }
     *ptr = p;
     return ret;
@@ -1369,6 +1371,8 @@ static int tcc_set_linker(TCCState *s, const char *option)
             ignoring = 1;
         } else if (link_option(option, "O", &p)) {
             ignoring = 1;
+        } else if (link_option(option, "export-all-symbols", &p)) {
+            s->rdynamic = 1;
         } else if (link_option(option, "rpath=", &p)) {
             copy_linker_arg(&s->rpath, p, ':');
         } else if (link_option(option, "enable-new-dtags", &p)) {
@@ -1390,7 +1394,7 @@ static int tcc_set_linker(TCCState *s, const char *option)
                 s->pe_subsystem = 1;
             } else if (!strcmp(p, "console")) {
                 s->pe_subsystem = 3;
-            } else if (!strcmp(p, "gui")) {
+            } else if (!strcmp(p, "gui") || !strcmp(p, "windows")) {
                 s->pe_subsystem = 2;
             } else if (!strcmp(p, "posix")) {
                 s->pe_subsystem = 7;