plowdown, plowprobe (process_url): add 2 more banned extension for file link list arguments

Simple sanity check. Just in case user confuse with plowup...

Plowshare3 fix: process_item() in plowprobe was using bash regex, use a switch/case instead (like in plowdown).
core.sh: add 'f' and 'F' type for argument parsing

f: check for filename with read access
F: check for executable (and consider $PATH)

Changes:
- process_options: a little bit more of docs
- in process_core_option, process_all_modules_options, process_module_options, drop useless ||return (because this is the last statement)
- check_argument_type: cosmetics, change error message
- check_argument_type: fix wrong test (it's was actually a bug but with no effect)

Note: Tilde expansion is not performed in "--option=~/foo". Use two arguments "--option" "~/foo" and bash will expand for us.
plowdown, plowup: use new 'f' & 'F' command-line argument type

Now used in --captchaprogram, --cookies, --run-before, --run-after.
Also updated man documentation for --captchaprogram.
plow*: command-line bug when using --

Order was wrong when double dash was used.

Also fix bad log_error message:

$ plowup ziddu -- foo
plowup: unsupported module (ziddu)