Don't complete multi-state entries until they're actually done.

[mtstickney]

When matching a multi-state entry, we need to differentiate between
determining whether the entry matches, and completing that state. A multi-
state entry should only be completed when we've reached the last of its
states. This is accomplished by returning the the completion function or
nil as the the primary value from check-table-entry, and a secondary value
indicating whether the entry matched.

Currently, the completion function is always called when the entry matches,
which manifests as the following bug (using interpol syntax for strings):

* (cl-csv:read-csv:read-csv #?|1,2,"abc"\r\n| :trim-outer-whitespace nil)

debugger invoked on a CL-CSV:CSV-PARSE-ERROR in thread
#<THREAD "main thread" RUNNING {10005185B3}>:
  whitespace after quoted data thats not supposed to be trimmed #<CSV-READER LINE-IDX:0 CHARACTER-LINE-IDX:10 CHARACTER-IDX:10 "abc" {1001D703F3}> 

Type HELP for debugger help, or (SB-EXT:EXIT) to exit from SBCL.

restarts (invokable by number or by possibly-abbreviated name):
  0: [ABORT] Exit debugger, returning to top level.

(CL-CSV:CSV-PARSE-ERROR "whitespace after quoted data thats not supposed to be trimmed ~a ~a" #<CL-CSV::CSV-READER LINE-IDX:0 CHARACTER-LINE-IDX:10 CHARACTER-IDX:10 "abc" {1001D703F3}> #\Return)
   source: (ERROR 'CSV-PARSE-ERROR :FORMAT-CONTROL MSG :FORMAT-ARGS ARGS)