Fix logic error in process_exited(s::ProcessChain)
Merge pull request #4791 from JuliaLang/cjh/process_exited_logic

Fix logic error in process_exited(s::ProcessChain)
Second attempt to fix logic error in process status functions

Fix #4791.
remove _MISSING value in dict. closes #4802
document scope of keyword and optional argument default values. closes #4795
fix #4801, keyword arg in constructor on first call
fix some of the issues in #4807, arrays of Complex{T<:Real}
improve matmul result type. closes #4807, closes #4796
Merge remote-tracking branch 'upstream/master'.  Preparing to update error messages to remove commas.