core.sh (captcha_process): 9kweu define maxtimeout parameter

10+6+6+7+7+8+8+9+9+10+10+20+20+20+30+30=210
In worst case, last request would get timeout error.

+ fix error range

Credits go to codeworld for original patch.
Fixes issue 921.
rapidshare (download): uri_decode (final) filename

Fixes issue 920.
uploaded.net (upload): add sanity check

Fixes issue 923.
plowdown: when probing for redirection use curl's --head switch
(instead of --include)
mediafire (download): fix captcha handling, second try

see also: issue 922 and commit 5aeeeadd6b152dc9991fceb4891baa7f5ef3561b