[mod_auth] permit specifying ldap DN; skip search (fixes #1248)
[mod_auth] permit specifying ldap DN; skip search (fixes #1248)

If auth.backend.ldap.filter begins with ',', then concatenate
uid=<username> with the 'filter' value to form the DN instead of using
ldap_search to query LDAP for the DN for the username, applying the
provided filter.

x-ref:
  "Allow User-DN to be supplied in the configuration rather than searching"
  https://redmine.lighttpd.net/issues/1248

gstrauss/lighttpd1.4,lighttpd/lighttpd1.4,lighttpd/lighttpd1.4,PatchyFog/lighttpd1.4-mbedtls,PatchyFog/lighttpd1.4-mbedtls,lighttpd/lighttpd1.4,PatchyFog/lighttpd1.4-mbedtls,PatchyFog/lighttpd1.4-mbedtls,lighttpd/lighttpd1.4,gstrauss/lighttpd1.4,gstrauss/lighttpd1.4,lighttpd/lighttpd1.4,gstrauss/lighttpd1.4,PatchyFog/lighttpd1.4-mbedtls,PatchyFog/lighttpd1.4-mbedtls,gstrauss/lighttpd1.4,PatchyFog/lighttpd1.4-mbedtls,lighttpd/lighttpd1.4,gstrauss/lighttpd1.4