Modify loop unwinding and general cleanup in Makefile.common

[markrtuttle]

This pull request removes conditional invocations of goto-instrument. The command goto-instrument is idempotent when called with no flags (it just copies the file), so checking whether a flag is set before calling goto-instrument is unnecessary and complicates the makefile.common.

This pull request also modifies when loop unwinding is done. It moves all loop unwinding of loops specified by UNWINDSET back to the invocations of cbmc (no more early invocation of goto-instrument to do it), but it does introduce a new EARLY_UNWINDSET to support selective loop unwinding for the purpose of function and loop contract checking.

This pull request also cleans up some comments in makefile.common and advances the release to 2.2.

These changes have been checked against all starter kit proofs in github. The differences found were

• removal of most (but not all) of the 'unknown loop identifier' warnings that resulted from giving loop unwinding instructions to both goto-instrument and cbmc
• removal of a 'conflicting initializers' warning in derive_data_key
• off-by-one differences in coverage reports due to the fact that a do statement is no longer considered by cbmc to be contributing goto instructions
• many instances of a line considered "both" hit and missed are now considered just "hit" (unrolling causes a single line of code to contribute to many basic blocks, some unreachable if the loop is unwound an extra iteration by goto-instrument)

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[markrtuttle]

This should close issue #88