* take a stab at fixing #99. ignore invalid output before we see a real packet. the complication here was ensuring we always output a real packet in every flow so we don't actually lose valid errors.
* add ping packets to prime the parser (when in ignoreUntilValid mode)
* switch emain to using the bash from PATH, not defaulting to /bin/bash. not convinced this fixes#132 yet because the PATH set for MacOS applications is not set from .bash_profile or .zprofile. those seem to be set with launchctl setenv.
* move MacUserShell() func to shexec. use dscl UserShell output as bash path on MacOS when present. fixes#132
