output "SET RETURN VALUE" when displaying goto programs

[kroening]

To avoid the impression that the 'set return value' instruction alters
control flow, output "SET RETURN VALUE" instead of "RETURN" when displaying
goto programs.

• Each commit message has a non-empty body, explaining why the change was made.
• n/a 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.

[codecov]

Codecov Report

Merging #6435 (c65874f) into develop (e5db980) will increase coverage by 0.00%.
The diff coverage is 95.65%.

@@           Coverage Diff            @@
##           develop    #6435   +/-   ##
========================================
  Coverage    76.00%   76.00%           
========================================
  Files         1527     1527           
  Lines       164454   164463    +9     
========================================
+ Hits        125000   125008    +8     
- Misses       39454    39455    +1     

Impacted Files	Coverage Δ
src/solvers/smt2_incremental/smt_commands.h	100.00% <ø> (ø)
src/goto-symex/symex_dead.cpp	95.65% <94.44%> (-4.35%)	⬇️
src/goto-programs/goto_program.cpp	69.74% <100.00%> (ø)
...rc/solvers/smt2_incremental/smt_to_smt2_string.cpp	93.96% <100.00%> (ø)
...it/solvers/smt2_incremental/smt_to_smt2_string.cpp	100.00% <100.00%> (ø)

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