kvs: Fix corner case in which symlink points to root dir

[chu11]

Fix corner case in which a symlink in the kvs points to the root dir, i.e. ".".

I discovered this corner case while working on #1341. I realized users will probably often do:

mysymlink -> namespace/.

i.e. a symlink simply points to the root of another namespace. But root dirs were not handled at all within symlinks.

[garlick]

Oh good one. Yes, I would imagine the "guest KVS namespaces" described in RFC 16 would be linked (instead of "mounted") exactly like that.

[coveralls]

Coverage increased (+0.001%) to 78.779% when pulling efa543a on chu11:symlinkperiod into a7402b5 on flux-framework:master.

[codecov-io]

Codecov Report

Merging #1404 into master will increase coverage by <.01%.
The diff coverage is 66.66%.

@@            Coverage Diff             @@
##           master    #1404      +/-   ##
==========================================
+ Coverage   78.46%   78.47%   +<.01%     
==========================================
  Files         163      163              
  Lines       29981    29983       +2     
==========================================
+ Hits        23526    23528       +2     
  Misses       6455     6455

Impacted Files	Coverage Δ
src/modules/kvs/lookup.c	79.78% <66.66%> (+0.08%)	⬆️
src/common/libflux/request.c	88.46% <0%> (-1.29%)	⬇️
src/common/libutil/dirwalk.c	93.57% <0%> (-0.72%)	⬇️
src/common/libkvs/kvs_watch.c	90.98% <0%> (-0.43%)	⬇️
src/cmd/flux-module.c	85.06% <0%> (-0.31%)	⬇️
src/broker/overlay.c	74.45% <0%> (+0.31%)	⬆️
src/common/libutil/blobref.c	98.61% <0%> (+1.38%)	⬆️
src/common/libflux/attr.c	92% <0%> (+2%)	⬆️

[chu11]

code coverage of diff is bad, but its just 1 missed "impossible error" path.