[PATCH] implement keywords substitution in mod_dav_svn

Hello, dev!

[[[
Add "SVNKeywordSubstitution" per-directory (repository) mod_dav_svn
configuration parameter (default is "Off"). Implement keywords
substitution.

* subversion/mod_dav_svn/dav_svn.h
  (dav_svn__get_keyword_subst_flag): declare new config access function

* subversion/mod_dav_svn/mod_dav_svn.c
  (dav_svn__get_keyword_subst_flag): implement it
  (dir_conf_t): add new config flag
  (merge_dir_config): handle it properly during merge
  (SVNKeywordSubst_cmd): parse new per-directory config settings
  (cmds): declare new parameter processing command

* subversion/mod_dav_svn/repos.c
  (set_headers):
    If parameter SVNKeywordSubstitution is On, don't send
    "Content-length". We can't guess the size of expanded stream at
    the moment (..is that bad?)
  (deliver):
    If parameter SVNKeywordSubstitution is On, perform keywords
    substitution, like client-side utilities do.
]]]

Basis of usefullness:

I usually upgrade my software (Oracle PL/SQL packages) directly from
Subversion repository using SQL*Plus scripts with "@" commands:
 @http://example.com/svn/project/package.bdy?p=123
Additionally, I wish to make use of svn:keywords substitution in
PL/SQL sources. Unfortunately, substitution is implemented only in
client-side utilities, but not in the DAV module.
I also noticed some other people missing same feature, so I decided to
make a try :)

P.S. I've never submitted Subversion patches before, please excuse my
inconvenience in case something is wrong.