core.sh: clarify --captchamethod

This used by plowdown & plowup.

Changes:
- add "fb" method
- rename "prompt" => "x11"
- updated captcha_method_translate()

So internally, captcha_process() is dealing with:

Solving methods:
- nop
- prompt (user prompt)
- online (antigate or 9kweu or captchabrotherhood or deathbycaptcha)

Viewing methods:
- none (echo temporary image path)
- imgur
- view-x (display or feh or sxiv or qiv)
- view-fb (fim)
- view-aa (img2txt, tiv, aview)

On user point of view:

"--captchamethod" methods:
- none (= nop + none)
- imgur (= prompt + imgur)
- x11 (= prompt + view-x)
- nox (= prompt + view-aa)
- online (= online + none)
- fb (= prompt + view-fb)
download.sh (mark_queue): cosmetics, fix vim syntax highligh
rapidgator (download): fix, upstream modified
docs (man pages): add captchas options for plowup.1

Changes:
- typo fix
- update --captchamethod

Related commits:
revision 90160fb8ac2485ed01899a1dd449608a6a22dfc4
revision 47c0c330e81c72d5cb765c46a23f39c5ae1435b9
core.sh: clarify --captchamethod [take 2]

This is used by plowdown & plowup.

Changes:
- add "fb" method
- rename "prompt" => "x11"
- updated captcha_method_translate()

So internally, captcha_process() is dealing with:

Solving methods:
- none (nothing is done)
- prompt (user prompt: stdin)
- online (antigate or 9kweu or captchabrotherhood or deathbycaptcha)

Viewing methods:
- log (echo temporary image path with log_notice)
- imgur
- view-x (display or feh or sxiv or qiv)
- view-fb (fim)
- view-aa (img2txt, tiv, aview)

On user point of view:

"--captchamethod" methods:
- none (= none + log)
- imgur (= prompt + imgur)
- x11 (= prompt + view-x)
- nox (= prompt + view-aa)
- online (= online + log)
- fb (= prompt + view-fb)