Added script to auto-generate enums along with symbolic and English text.

Using this new script to auto-generate MSG_DROP_XXX constants and their
English descriptions from a template file.
Converted general statistics to automatic enum / description generation.
Use new LinkSourceFileAlias() jmake instruction to link files from if/gen.
Converted message types to automatic enum / description generation.
Generated stringification of enum now displays out-of-boundary values.
Moved enum description header at the top of the enum definition file.
Converted HTTP async and URL parsing errors to automatic enum generation.
Converted download mesh URL parsing errors to automatic enum generation.
Converted download ignore codes to automatic enum generation.
Converted XML parsing error codes to automatic enum generation.
Added the "stats drop" shell command to show dropped message statistics.

The legacy "stats" command becomes a shortcut for "stats general".