subids: support nsswitch

Closes #154

When starting any operation to do with subuid delegation, check
nsswitch for a module to use.  If none is specified, then use
the traditional /etc/subuid and /etc/subgid files.

Currently only one module is supported, and there is no fallback
to the files on errors.  Several possibilities could be considered:

1. in case of connection error, fall back to files
2. in case of unknown user, also fall back to files

etc...

When non-files nss module is used, functions to edit the range
are not supported.  It may make sense to support it, but it also
may make sense to require another tool to be used.

libsubordinateio also uses the nss_ helpers.  This is how for instance
lxc could easily be converted to supporting nsswitch.

Add a set of test cases, including a dummy libsubid_zzz module.  This
hardcodes values such that:

'ubuntu' gets 200000 - 300000
'user1' gets 100000 - 165536
'error' emulates an nss module error
'unknown' emulates a user unknown to the nss module
'conn' emulates a connection error ot the nss module

Changes to libsubid:

Change the list_owner_ranges api: return a count instead of making the array
null terminated.

This is a breaking change, so bump the libsubid abi major number.

Rename free_subuid_range and free_subgid_range to ungrant_subuid_range,
because otherwise it's confusing with free_subid_ranges which frees
    memory.

Run libsubid tests in jenkins

Switch argument order in find_subid_owners

Move the db locking into subordinateio.c

Signed-off-by: Serge Hallyn <serge@hallyn.com>

lib/prototypes.h

@@ -262,6 +262,75 @@ extern void motd (void);
 /* myname.c */
 extern /*@null@*//*@only@*/struct passwd *get_my_pwent (void);
 
+/* nss.c */
+#include <libsubid/subid.h>
+extern void nss_init(char *nsswitch_path);
+extern bool nss_is_initialized();
+
+struct subid_nss_ops {
+	/*
+	 * nss_has_any_range: does a user own any subid range
+	 *
+	 * @owner: username
+	 * @idtype: subuid or subgid
+	 * @result: true if a subid allocation was found for @owner
+	 *
+	 * returns success if the module was able to determine an answer (true or false),
+	 * else an error status.
+	 */
+	enum subid_status (*has_any_range)(const char *owner, enum subid_type idtype, bool *result);
+
+	/*
+	 * nss_has_range: does a user own a given subid range
+	 *
+	 * @owner: username
+	 * @start: first subid in queried range
+	 * @count: number of subids in queried range
+	 * @idtype: subuid or subgid
+	 * @result: true if @owner has been allocated the subid range.
+	 *
+	 * returns success if the module was able to determine an answer (true or false),
+	 * else an error status.
+	 */
+	enum subid_status (*has_range)(const char *owner, unsigned long start, unsigned long count, enum subid_type idtype, bool *result);
+
+	/*
+	 * nss_list_owner_ranges: list the subid ranges delegated to a user.
+	 *
+	 * @owner - string representing username being queried
+	 * @id_type - subuid or subgid
+	 * @ranges - pointer to an array of struct subordinate_range pointers, or
+	 *           NULL.  The returned array of struct subordinate_range and its
+	 *           members must be freed by the caller.
+	 * @count - pointer to an integer into which the number of returned ranges
+	 *          is written.
+
+	 * returns success if the module was able to determine an answer,
+	 * else an error status.
+	 */
+	enum subid_status (*list_owner_ranges)(const char *owner, enum subid_type id_type, struct subordinate_range ***ranges, int *count);
+
+	/*
+	 * nss_find_subid_owners: find uids who own a given subuid or subgid.
+	 *
+	 * @id - the delegated id (subuid or subgid) being queried
+	 * @id_type - subuid or subgid
+	 * @uids - pointer to an array of uids which will be allocated by
+	 *         nss_find_subid_owners()
+	 * @count - number of uids found
+	 *
+	 * returns success if the module was able to determine an answer,
+	 * else an error status.
+	 */
+	enum subid_status (*find_subid_owners)(unsigned long id, enum subid_type id_type, uid_t **uids, int *count);
+
+	/* The dlsym handle to close */
+	void *handle;
+};
+
+extern struct subid_nss_ops *get_subid_nss_handle();
+
+
 /* pam_pass_non_interactive.c */
 #ifdef USE_PAM
 extern int do_pam_passwd_non_interactive (const char *pam_service,