Sync parts of FreeBSD Process plugin with Linux

* Use PseudoTerminal to fix stdio handling / passthrough to the inferior
  process.
* Add log messages equivalent to the Linux ones.
* Port changes relating to process creation / termination.

This revision contains changes equivalent to (parts of) SVN revisions
109318 142384 166055 168503 169645 177116 182809.


git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@185442 91177308-0d34-0410-b5e6-96231b3b80d8