Make comments in main an init doxygen compatible

Fix typos where appropriate
Update license/copyright
Coinbases-in-mempool regression test

Immature coinbase spends are allowed in the memory pool if they can be mined in the next block.
They are not allowed in the memory pool if they cannot be mined in the next block.

This regression test tests those edge cases.
Merge pull request #5403

c5b390b Make comments in main an init doxygen compatible (Michael Ford)
Merge pull request #5407

9e56532 Coinbases-in-mempool regression test (Gavin Andresen)
Merge remote-tracking branch 'upstream/master'