Add 'swupd clean' command

This command removes old files from statedir. The interface is a bit
opaque so there will be room to modify the policy later.

By default it will delete staged files that are older than
DAYS_TO_KEEP_FILES. For manifests this heuristic is not good as older
manifests might still be used for the current OS version being run, so
also ensure that those do not get deleted. This prevents 'swupd
search' to redownload files.

The default behavior should be suitable to use in combination with
automatic updates, to control the size of state dir. There is also an
--all option to remove all the state files regardless of dates and
usage.

To avoid "disasters" in case some paths are set wrong, the command
explicitly delete patterns of files create by swupd.

Makefile.am

@@ -10,6 +10,7 @@ bin_PROGRAMS = swupd
 swupd_SOURCES = \
 	src/autoupdate.c \
 	src/check_update.c \
+	src/clean.c \
 	src/clr_bundle_add.c \
 	src/clr_bundle_ls.c \
 	src/clr_bundle_rm.c \

include/swupd-internal.h

@@ -11,5 +11,6 @@ extern int verify_main(int argc, char **argv);
 extern int check_update_main(int argc, char **argv);
 extern int search_main(int argc, char **argv);
 extern int info_main(int argc, char **argv);
+extern int clean_main(int argc, char **argv);
 
 #endif