[tautschnig]

The fesetround library model unconditionally set __CPROVER_rounding_mode
to round-to-nearest (0) and returned 0 for any argument not matching a
rounding-direction macro. This violates:

• C11 7.6.3.2p2: if the argument is not equal to the value of a
  rounding direction macro, the rounding direction is not changed.
• C11 7.6.3.2p3: fesetround returns zero if and only if the requested
  rounding direction was established.

The model now leaves _CPROVER_rounding_mode unchanged and returns -1
when the argument does not match any FE* macro.

Fixes #9326

Found by Jeroen Robben's specification-conformance pipeline (https://reports.robben.io/methodology/).

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