(svn r7565) -Codechange: Rework DEBUG functionality. Look for appropiate debugging levels to
use in debug.h. grfmsg() is now used as a specific debug-function for grf.

(svn r6816) -Codechange: Some coding style, variable localization, const correctness.
-Codechange: 'quited' is not a word, use has_quit instead for NetworkClientState

(svn r6750) -Codechange: Juggle around some header includes as they're only used when
networking is enabled.

(svn r4413) -Fix: fixed a bug which pushed the client back to the main menu when a
server is in the mainserver-list which sends out illegal signals. Many
tnx to 'test' for finding and isolating the problem.
-Fix: also specify the problem a bit better

(svn r3564) Several smaller changes:
- Don't treat non-booleans as booleans
- Reduce variable scope
- Bracing
- Use DeMorgan's law to make conditionals easier to read
- if cascade -> switch
- Replace some magic numbers by symbolic names
- Avoid assignments within other statements

(svn r1389) -Add: [Network] Added packet protection. No longer a client or server
reads beyond the size of the packet
-Fix: [Network] A server no longer crashes when a client sends an
invalid DoCommand, but drops the client instead.

(svn r1168) -Cleanup: [Network] Cleaned the network code a bit. Added 'const'
and 'void' where needed, prefixed all functions, typedefs and global
vars with 'Network' and organized all externals nicely.