core.sh (check_exec): better version (use command builtin)
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