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.

llvm-svn: 185442