filepost: add probe support
filepost (download): fix login (missing command substitution)
filepost: also accept short URLs (fp.io)
core.sh (uri_encore, uri_encode_strict, uri_decode): one single expression given to sed

+ add %21 (!) in uri_decode
core.sh (uri_encode_strict): fix bug, replace percent sign (%) first
plowup: add captchas command-line options (like plowdown)

Add --captchamethod, --captchaprogram, --9kweu, --antigate, --captchabhood and --deathbycaptch switches
Documentation will come later.

Like plowdown, $ERR_CAPTCHA is eligible for retry policy.
See revision e5648e2b37049e3466cd6ede625ccf3b3ef0bdd2