Add MS-WDV support

MS-WDV specification:
https://learn.microsoft.com/en-us/openspecs/windows_protocols/ms-wdv

The changes introduces the DAVMSext directive, which is used to 
enable MS-WDV: DAVMSext +WDV

dav_get_timeout_string() is introduced as a variant of dav_get_timeout().
The former parses a string, the later parse the Timeout HTTP header.



git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1907608 13f79535-47bb-0310-9956-ffa450edef68
