[Fix] lua_selectors: reject selectors that parse only as a prefix

[Fix] lua_selectors: reject selectors that parse only as a prefix

The selector grammar was not anchored to the end of input, so lpeg
matched the longest valid prefix and silently dropped the rest of the
string. E.g. 'from("smtp"):domain:lower' (a second ':' cannot parse)
was accepted and evaluated as 'from("smtp"):domain', and any trailing
garbage after a selector or a ';' list element was ignored. All
consumers (multimap, rbl, ratelimit, settings, reputation, the
controller selector check used by the WebUI) reported such selectors
as valid while evaluating only the prefix.

Append an lpeg.Cp() capture after the grammar (plus optional trailing
whitespace) and make parse_selector require the whole input to be
consumed, logging the position and the unparsed tail otherwise.

User-visible change: selectors that previously loaded thanks to the
silent truncation now fail configuration load with an error pointing
at the offending token. Such selectors were never evaluating as
written, so failing loudly is the correct behaviour.