Make the sp_lstchg shadow field reproducible.

The third field in the /etc/shadow file (sp_lstchg) contains the date of
the last password change expressed as the number of days since Jan 1, 1970.
As this is a relative time, creating a user today will result in:

   username:17238:0:99999:7:::

whilst creating the same user tomorrow will result in:

    username:17239:0:99999:7:::

This has an impact for the Reproducible Builds[0] project where we aim to
be independent of as many elements the build environment as possible,
including the current date.

This patch changes the behaviour to use the SOURCE_DATE_EPOCH[1]
environment variable (instead of Jan 1, 1970) if valid.

 [0] https://reproducible-builds.org/
 [1] https://reproducible-builds.org/specs/source-date-epoch/

Signed-off-by: Chris Lamb <lamby@debian.org>

lib/prototypes.h

@@ -179,6 +179,9 @@ extern int getrange (char *range,
                      unsigned long *min, bool *has_min,
                      unsigned long *max, bool *has_max);
 
+/* gettime.c */
+extern time_t gettime ();
+
 /* get_uid.c */
 extern int get_uid (const char *uidstr, uid_t *uid);