Add check for allowed Window PoSt proof type at miner creation

[anorth]

In #1345 we unintentionally lost a check that prevented creation of new miners with proofs types that are not supported by the network. Such miners can't do any harm (they can never commit a sector) but it's still better to prevent their creation up front.

[codecov-commenter]

Codecov Report

Merging #1421 (46a1937) into master (0dfdf81) will increase coverage by 0.2%. The diff coverage is 77.7%.

@@           Coverage Diff            @@
##           master   #1421     +/-   ##
========================================
+ Coverage    69.9%   70.2%   +0.2%     
========================================
  Files          72      72             
  Lines        7773    7779      +6     
========================================
+ Hits         5438    5463     +25     
+ Misses       1444    1434     -10     
+ Partials      891     882      -9