ask-password: add support for caching passwords in the kernel keyring

This adds support for caching harddisk passwords in the kernel keyring
if it is available, thus supporting caching without Plymouth being
around.

This is also useful for hooking up "gdm-auto-login" with the collected
boot-time harddisk password, in order to support gnome keyring
passphrase unlocking via the HDD password, if it is the same.

Any passwords added to the kernel keyring this way have a timeout of
2.5min at which time they are purged from the kernel.

src/basic/strv.h

@@ -40,7 +40,7 @@ void strv_clear(char **l);
 char **strv_copy(char * const *l);
 unsigned strv_length(char * const *l) _pure_;
 
-int strv_extend_strv(char ***a, char **b);
+int strv_extend_strv(char ***a, char **b, bool filter_duplicates);
 int strv_extend_strv_concat(char ***a, char **b, const char *suffix);
 int strv_extend(char ***l, const char *value);
 int strv_extendf(char ***l, const char *format, ...) _printf_(2,0);
@@ -80,6 +80,7 @@ char *strv_join_quoted(char **l);
 
 char **strv_parse_nulstr(const char *s, size_t l);
 char **strv_split_nulstr(const char *s);
+int strv_make_nulstr(char **l, char **p, size_t *n);
 
 bool strv_overlap(char **a, char **b) _pure_;