http_ntlm: Renamed from curl_ntlm.[c|h]

Renamed the header and source files for this module as they are HTTP
specific and as such, they should use the naming convention as other
HTTP authentication source files do - this revert commit 260ee6b7.

Note: We could also rename curl_ntlm_wb.[c|h], however, the Winbind
code needs separating from the HTTP protocol and migrating into the
vauth directory, thus adding support for Winbind to the SASL based
protocols such as IMAP, POP3 and SMTP.