-Fix (#162): sonic blasts did not trigger enemy counter attack
-Codechange: coding style
-Cleanup: simplify String_Decompress, add const
-Fix: Remove strdup() declaration for OS X

Removes warnings :
OpenDUNE/src/os/strings.h:91: warning: redundant redeclaration of ‘strdup’
/usr/include/string.h:116: warning: previous declaration of ‘strdup’ was here