Split shell code out to support optional inclusion

[skliper]

Is your feature request related to a problem? Please describe.
Execute in shell functionality is undesirable for some security conscious applications.

Describe the solution you'd like
Allow for optional inclusion (like network option?), respond as not implemented when not included.

Describe alternatives you've considered
Discussed security at length, diverse use cases require flexibility for user to decide.

Additional context
POSIX issue found in LGTM

Requester Info
Jacob Hageman - NASA/GSFC