miscellaneous mod_dtcl documentation

## Other mod_dtcl Documentation

### Internals

- Read the code!
- Initialization
  When Apache is started, (or when child Apache processes are started if a threaded Tcl is used), `tcl_init_stuff` is called, which creates a new interpreter, and initializes various things, like the `apache_channel` channel system. The caching system is also set up, and if there is a GlobalScript, it is run.
- Achan/apache_channel
  The "Apache Channel" system was created so that it is possible to have an actual Tcl channel that we could redirect standard output to. This lets us use, for instance, the regular "puts" command in .ttml pages. It works by creating commands that write to memory that is slated to be sent to the client.
- Page parsing/execution
  In `send_parsed_file` Each .ttml file is loaded and run within its own namespace. No new interpreter is created for each page. This lets you share variables, and most importantly, loaded modules, from a common parent (such as one of the InitScripts). When a file is loaded, it is transformed into a Tcl script by putting everything outside of <? and ?> into large hputs statements. When the script is complete, it is then inserted into the cache, for future use. In fact, if the file modification information doesn't change, mod_dtcl will execute the cached version of the script the next time it is encountered.
- Binary data
  mod_dtcl is capable of outputing binary data, such as images, or loading binary data with 'include'.

### Upgrading From Older (< 0.9.3) Versions

- New tags
  As of version 0.9.4, mod_dtcl uses <? and ?> instead of <+ and +> to delimit sections of Tcl code. By default, dtcl is still compiled with code to parse the <? ?> tags, but it will run faster if `USE_OLD_TAGS` is set to 0 in `mod_dtcl.h`. You can automatically change your .ttml files to the new format by using the `newtags.sh` script in the contrib/ directory. Run it at the top level of your DocumentRoot.
- headers setcookie
  The `headers setcookie` command now uses the `-name` and `-value` flags for those arguments, whereas this was not necessary in the past.