Compile fixes for buidbots

A few changes to variable declarations so that the buildbots are happy:

• Unsigned var vs. signed var comparisons -- changed some signed ints to unsigned
  • I'm pretty sure this is safe from overflow. The affected vars already check for <= 0 so that they're prevented from overflowing.
• Mixed D&C -- move declarations to top of code blocks
• Some int changed to INT32
  • This one doesn't really matter -- there are a few places in code where int is used -- but it helps portability for other architectures.
• Fix strncpy(nodenum, msg+3, 5) calls so that the copy length (5) does not overflow the malloc size of nodenum (3)

Buildbot result before this merge:

• Unsigned vs. signed comparisons: (local build; build the original branch with ERRORMODE=1 to see)
• Mixed D&C: https://circleci.com/gh/mazmazz/SRB2/366
• strncpy overflow: https://travis-ci.org/mazmazz/SRB2/jobs/469284741

Buildbot result ater this merge:

• https://travis-ci.org/mazmazz/SRB2/builds/469322912