MPS: posix_main.c specs that are currently possible

[peterohanley]

Describe your changes

Implented specs for all functions except for the following:

• send_actuation_command: uses malloc, Write CN library models as needed for VERSE examples  VERSE-Toolchain#84
• read_actuation_command: global variables and a scope issue, [CN] not clear how to use static local variables rems-project/cerberus#353
• update_sensor_errors: contains a 2-level and 3-level array with the same variables used as indices in both, [CN] Difficult to work with multiple multidimensional arrays that share indices rems-project/cerberus#357
• update_sensors: also [CN] Difficult to work with multiple multidimensional arrays that share indices rems-project/cerberus#357
• clear_screen: need to claim that stdin is a valid pointer while using accesses. Could just use take but it's a global. This is also affected by the spec chosen for fileno
• read_rts_command: uses sscanf and uses it at multiple types, [CN] Support stdio.h (skip variadic functions, define _IO_FILE type) rems-project/cerberus#301

A solution to rems-project/cerberus#358 will also be needed instead of the stub declarations used currently.

Issue ticket number and link

#55

Checklist before requesting a review

• I have performed a self-review of my code
• My code matches the coding standards and I have ran the appropriate linters
• I included documentation updates for my code
• I extended the test suite and the tests run by the CI to cover my code
• I assigned a Milestone to this PR
• I assigned this PR to a Project
• I assigned this PR appropriate Labels

[podhrmic]

@peterohanley what is the relation of this branch and 11-implement-mps?

[podhrmic]

Due date 12/4