libjobspec: ensure task count has exactly one key

[sam-maloney]

Problem: the desired number of tasks is ambiguous if more than one key is set in tasks.count

Fail with an error message if multiple keys are found in the jobspec

This is based on the proposed amendment to RFC 14 in flux-framework/rfc#441

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 75.7%. Comparing base (da6156a) to head (992bc9c).

Additional details and impacted files

@@          Coverage Diff           @@
##           master   #1343   +/-   ##
======================================
  Coverage    75.7%   75.7%           
======================================
  Files         112     112           
  Lines       16410   16412    +2     
======================================
+ Hits        12426   12428    +2     
  Misses       3984    3984           

Files with missing lines	Coverage Δ
resource/libjobspec/jobspec.cpp	87.0% <100.0%> (+<0.1%)	⬆️