fstab-generator: add x-systemd.requires and x-systemd.requires-mounts-for

Currently we have no way how to specify dependencies between fstab
entries (or another units) in the /etc/fstab. It means that users are
forced to bypass fstab and write .mount units manually.

The patch introduces new systemd fstab options:

x-systemd.requires=<PATH>

 - to specify dependence an another mount (PATH is translated to unit name)

x-systemd.requires=<UNIT>

 - to specify dependence on arbitrary UNIT

x-systemd.requires-mounts-for=<PATH ...>

 - to specify dependence on another paths, implemented by
   RequiresMountsFor=. The option may be specified more than once.

For example two bind mounts where B depends on A:

 /mnt/test/A    /mnt/test/A     none    bind,defaults
 /mnt/test/A    /mnt/test/B     none    bind,x-systemd.requires=/mnt/test/A

More complex example with overlay FS where one mount point depends on
"low" and "upper" directories:

 /dev/sdc1   /mnt/low    ext4     defaults
 /dev/sdc2   /mnt/high   ext4     defaults
 overlay     /mnt/merged overlay  lowerdir=/mnt/low,upperdir=/mnt/high/data,workdir=/mnt/high/work,x-systemd.requires-mounts-for=/mnt/low,x-systemd.requires-mounts-for=mnt/high

https://bugzilla.redhat.com/show_bug.cgi?id=812826
https://bugzilla.redhat.com/show_bug.cgi?id=1164334

src/shared/fstab-util.c

@@ -125,6 +125,36 @@ answer:
         return !!n;
 }
 
+int fstab_extract_values(const char *opts, const char *name, char ***values) {
+        _cleanup_strv_free_ char **optsv = NULL, **res = NULL;
+        char **s;
+
+        assert(opts);
+        assert(name);
+        assert(values);
+
+        optsv = strv_split(opts, ",");
+        if (!optsv)
+                return -ENOMEM;
+
+        STRV_FOREACH(s, optsv) {
+                char *arg;
+                int r;
+
+                arg = startswith(*s, name);
+                if (!arg || *arg != '=')
+                        continue;
+                r = strv_extend(&res, arg + 1);
+                if (r < 0)
+                        return r;
+        }
+
+        *values = res;
+        res = NULL;
+
+        return !!*values;
+}
+
 int fstab_find_pri(const char *options, int *ret) {
         _cleanup_free_ char *opt = NULL;
         int r;

src/shared/fstab-util.h

@@ -28,6 +28,8 @@
 int fstab_filter_options(const char *opts, const char *names,
                          const char **namefound, char **value, char **filtered);
 
+int fstab_extract_values(const char *opts, const char *name, char ***values);
+
 static inline bool fstab_test_option(const char *opts, const char *names) {
         return !!fstab_filter_options(opts, names, NULL, NULL, NULL);
 }