Add interface to lcb_cntl_string

This is a very simple interface. It is currently undocumented but should
be much easier to follow and use than the numeric _cntl interface;
however the numeric _cntl interface might still be preferrable for
settings which don't have a distinct mapping.

Change-Id: I281879efd22bcf8e9e4337b29462cff5636c78ce
Reviewed-on: http://review.couchbase.org/44151
Tested-by: Mark Nunberg <mnunberg@haskalah.org>
Reviewed-by: Volker Mische <volker.mische@gmail.com>