core.sh (parse_attr, parse_all_attr): fix unquoted attributes

/ is a perfectly legal character for an unquoted attribute value.

According to HTML standard (don't include XHTML because it wants only double quotes):

U+0022 QUOTATION MARK characters (")
U+0027 APOSTROPHE characters (')
U+003D EQUALS SIGN characters (=)
U+003C LESS-THAN SIGN characters (<)
U+003E GREATER-THAN SIGN characters (>)
U+0060 GRAVE ACCENT characters (`)

Regression introduced in commit:
core.sh (parse_* functions): escaping / is not mandatory anymore (fix 2/2)
revision 67d0835b4a8bcbdfdd863873d7b6a1700d4fb96e
docs (man pages): add missing $ERR_BAD_COMMAND_LINE exit code for plowprobe.1
ziddu: new module with download, upload, delete & probe support

Original patch provided by stone.bronson

Fixes issue 951.
plowdown, plowprobe (process_url): add 2 more banned extension for file link list arguments

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