Fix #354, Shell related API separated for optional implementation

[skliper]

Describe the contribution
Adds a portable OS_ShellOutputToFile_Impl no-shell implementation following the no-network model
Splits posix/rtems/vxworks OS_ShellOutputToFile_Impl into a separate osshell.c (was in osfileapi.c)
Defaults to use the not implemented version (to match certification target)
Added unit test wrapper
Fix #354

Testing performed
Steps taken to test the contribution:

1. Built for posix with and with not implemented version
2. CI test with not implemented - https://travis-ci.com/github/skliper/cFS/builds/160380448

NOTE - build for vxworks attempted, but ran into other issues introduced since last check on that system (elf2cfetbl casts missing to match format)

Expected behavior changes
Command to run in shell and output to fill will fail with default (not implemented) setting. Open issue to resolve is nasa/cFE#484

System(s) tested on

• Hardware: cFS Dev Server

• OS: Ubuntu 18.04

• Versions: Master bundle w/ this commit on top of Fix #581, Propagate the OSAL compile definitions to CFE build cFE#585, Fix #121, Unify CFE PSP with OSAL BSP PSP#149, Fix #261, #312, and #362, OSAL build cleanup (multiple issues) #404

• Recommend build/test on RTEMS

• Resolve unrelated issue for VxWorks and complete test

Additional context
None

Third party code
None

Contributor Info - All information REQUIRED for consideration of pull request
Jacob Hageman - NASA/GSFC

[astrogeco]

CCB 20200415 - APPROVED, @jphickey to check RTEMS, will need to be reconciled #285

[skliper]

CCB 20200415 - APPROVED, will need rebase prior to merge to IC

[skliper]

Rebased on master