system_data_types.7: Add 'uid_t'

Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>

man7/system_data_types.7

@@ -741,6 +741,38 @@ See also:
 .BR adjtime (3),
 .BR futimes (3),
 .BR timeradd (3)
+.\"------------------------------------- uid_t ----------------------/
+.TP
+.I uid_t
+.IP
+Include:
+.IR <sys/types.h> ;
+or
+.I <pwd.h>
+or
+.I <signal.h>
+or
+.I <stropts.h>
+or
+.I <sys/ipc.h>
+or
+.I <sys/stat.h>
+or
+.IR <unistd.h> .
+.IP
+A type used to hold user IDs.
+According to POSIX,
+this shall be an integer type.
+.IP
+Conforming to: POSIX.1-2001 and later.
+.IP
+See also:
+.BR chown (2),
+.BR getuid (2),
+.BR geteuid (2),
+.BR getresuid (2),
+.BR getpwnam (2),
+.BR credentials (7)
 .SH NOTES
 The structures described in this manual page shall contain,
 at least, the members shown in their definition, in no particular order.