instaweb: refactor httpd code into separate files

Taking a cue from commit bc7a96a (mergetool--lib: Refactor tools into
separate files, 2011-08-18), refactor the code for each HTTP daemon
supported by git-instaweb into a separate files.

Each supported daemon has a file in $GIT_EXEC_PATH/httpds/ and new
daemons can be added simply by creating a new file rather than editing
the git-instaweb.sh script.

Signed-off-by: John Keeping <john@keeping.me.uk>