lib/adds.[ch]: Add addsl() and addsl3()

These functions add 2 or 3 longs, saturating to LONG_{MIN,MAX} instead
of overflowing.

Cc: Tobias Stoeckmann <tobias@stoeckmann.org>
Signed-off-by: Alejandro Colomar <alx@kernel.org>

lib/Makefile.am

@@ -23,6 +23,8 @@ libshadow_la_CFLAGS = $(LIBBSD_CFLAGS) $(LIBCRYPT_PAM) $(LIBSYSTEMD)
 
 libshadow_la_SOURCES = \
 	addgrps.c \
+	adds.c \
+	adds.h \
 	age.c \
 	agetpass.c \
 	agetpass.h \