Cleanup fmod vs IEEE remainder

[tautschnig]

1. Add tests for both the C and Java front-end.
2. Make sure we support both variants of the modulus operator on floating-point values in the back-ends: the SMT back-end only supported remainder (and only for FPA mode), the propositional back-end did not distinguish between fmod and IEEE remainder.

• Each commit message has a non-empty body, explaining why the change was made.
• Methods or procedures I have added are documented, following the guidelines provided in CODING_STANDARD.md.
• n/a The feature or user visible behaviour I have added or modified has been documented in the User Guide in doc/cprover-manual/
• Regression or unit tests are included, or existing tests cover the modified code (in this case I have detailed which ones those are in the commit message).
• n/a My commit message includes data points confirming performance improvements (if claimed).
• My PR is restricted to a single feature or bugfix.
• n/a White-space or formatting changes outside the feature-related changed lines are in commits of their own.

[tautschnig]

Asking for help from @martin-cs: the remaining issue is that we need an ability to round-to-integer (roundToIntegral in SMT2) in our back-ends.