No implicit <Directory> entry for ScriptAlias path

Support for ScriptAlias currently implicitly and automatically creates
<Directory> configuration directives for the path component in a given
ScriptAlias directive. These are not useful or needed, according to the
mod_alias documentation. Fixes issue #486.

This change also indirectly relates to #487 because the implicitly
created <Directory> entry for each ScriptAlias path also had
`SSLOptions +StdEnvVars` set for that directory.