CMessageHeader sanity changes

- Remove spurious `pchCommand[1] = 1` in CMessageHeader()
- Make sure that pchCommand is zero-padded if length is shorter than
  COMMAND_SIZE
- Use strnlen to determine length of pcmCommand in GetCommand
Use a typedef for monetary values
remove dead/unused code in walletdb.cpp

- closes #5021
qt: Register CAmount metatype

This allows sending the type over queued connections.
Also normalize signal/slot names.
Merge pull request #4234

c122f55 qt: Register CAmount metatype (Wladimir J. van der Laan)
a372168 Use a typedef for monetary values (Mark Friedenbach)
doc: add make install to build-osx.md and build-unix.md
Merge pull request #5022

c375b5c remove dead/unused code in walletdb.cpp (Philip Kaufmann)
travis: enable windows tests
Merge pull request #5023

e9f3fa7 travis: enable windows tests (Cory Fields)
Merge pull request #4940

62e5f8f CMessageHeader sanity changes (Wladimir J. van der Laan)