Fix mxb_strerror

The function should not be an inline function with a static variable. This
appears to cause problems on at least Debian Wheezy and is likely to cause
odd behavior on other platforms.

Also renamed the file to <maxbase/string.h> to better mirror how the
<string.h> file behaves.

maxutils/maxbase/src/CMakeLists.txt

@@ -3,6 +3,7 @@ add_library(maxbase STATIC
   eventcount.cc
   logger.cc
   stopwatch.cc
+  string.cc
   stacktrace.cc
   )
 set_target_properties(maxbase PROPERTIES VERSION "1.0.0" LINK_FLAGS -Wl,-z,defs)