Cleanup 'undefined-function' options to use 'generate-function-body'

[chrisr-diffblue]

Once #1889 is merged, --undefined-function-is-assume-false can be deprecated and its functionality replaced with the combination of --generate-function-body '.*' --generate-function-body-options assume-false. Likewise undefined_function_abort_path could also be replaced with the --generate-function-body mechanism. These and any similar options should be cleaned up to use the new mechanism.

[chrisr-diffblue]

@tautschnig - I hope this issue captures the thoughts you had on cleanups that followed the work in #1889 but please add anything else that I may have missed.

[martin-cs]

#1899 has been merged... :-)