fts: introduce FTS_VERBATIM

fts: introduce FTS_VERBATIM

This gives clients the option to disable stripping of trailing slashes
from input path names during fts_open initialization.

The recent change v0.0-7611-g3a9002d that made fts_open strip trailing
slashes from input path names had a negative impact on findutils that
relies on the old fts_open behavior to implement POSIX requirement that
each path operand of the find utility shall be evaluated unaltered as it
was provided, including all trailing slash characters.

* lib/fts_.h (FTS_VERBATIM): New bit flag.
(FTS_OPTIONMASK, FTS_NAMEONLY, FTS_STOP): Adjust.
* lib/fts.c (fts_open): Honor it.