Fix #329, resolve osfileapi.c lgtm issue

[avan989]

Describe the contribution
Fixes #329
Resolve osfileapi.c lgtm issue.

Testing performed
Steps taken to test the contribution:

1. Build dummy repository against lgtm
2. Verify it fixes.

Expected behavior changes
A clear and concise description of how this contribution will change behavior and level of impact.

• execl() will only execute for /bin/sh.

System(s) tested on:

• Hardware
• Ubuntu 18.04
• CFE 6.6

Contributor Info
Anh Van, NASA Goddard

[skliper]

Considering removing the shell code completely (major security issue). Will discuss in CCB.

[skliper]

CCB 20190108 - Decision to retain functionality for now, discussed potential optional compile (like network code). The change in this pull request should be reviewed by CCB to improve security.

[skliper]

Likely can remove the shell variable, if no longer applies now that it's hard coded.

Suggest wait for CCB review (may have other changes)

[skliper]

CCB 20200115 - Discussion is to split this shell code out, and optionally included

[skliper]

See #354, which will leave as is but make optional.