idmap.c for ldap caching and configuration

struct idmap_context contains configuration data (struct idmap_config), a cache for users, and a cache for groups.  idmap_context is declared in idmap.c, and only available as an opaque pointer (nfs41_idmapper) elsewhere.  similarly, Winldap.h is only included by idmap.c, and not needed elsewhere

nfs41_idmap_create() allocates the idmap_context, loads the configuration from file, and calls ldap_init().  it does not call ldap_connect(); we'll still be able to start the daemon if ldap isn't configured, or the ldap server is down.  calling ldap_connect() is optional, as any ldap operation that requires a connection will establish it internally.  this behavior, along with the LDAP_OPT_AUTO_RECONNECT option (defaults to on), means that we shouldn't have to maintain a separate connection for each thread

nfs41_idmap_*() functions return windows errors codes.  LDAP_RETCODEs are mapped to windows errors with LdapMapErrorToWin32()

the user and group caches share a common generic interface in struct idmap_cache, which uses a linked list for storage, and protects access with a SRWLOCK.  expiration of cache entries can be adjusted by the config option 'cache_ttl'

struct config_option g_options[] is a table of available config options and their default values.  this patch adds a 'ms-nfs41-idmap.conf' file with all possible options set to default values, and commented out.  the daemon expects to find this file under c:\etc\, and won't start if it can't be opened or parsed

Signed-off-by: Casey Bodley <cbodley@citi.umich.edu>

ms-nfs41-idmap.conf

@@ -0,0 +1,20 @@
+# ldap server information
+#ldap_hostname="localhost"
+#ldap_port="389"
+#ldap_version="3"
+#ldap_timeout="5"
+
+# ldap schema information
+#ldap_base="cn=localhost"
+
+#ldap_class_users="user"
+#ldap_class_groups="group"
+
+#ldap_attr_username="cn"
+#ldap_attr_groupname="cn"
+#ldap_attr_gssAuthName="gssAuthName"
+#ldap_attr_uidNumber="uidNumber"
+#ldap_attr_gidNumber="gidNumber"
+
+# caching configuration
+#cache_ttl="60"