install: follow unit file symlinks in /usr, but not /etc when looking for [Install] data

Some distributions use alias unit files via symlinks in /usr to cover
for legacy service names. With this change we'll allow "systemctl
enable" on such aliases.

Previously, our rule was that symlinks are user configuration that
"systemctl enable" + "systemctl disable" creates and removes, while unit
files is where the instructions to do so are store. As a result of the
rule we'd never read install information through symlinks, since that
would mix enablement state with installation instructions.

Now, the new rule is that only symlinks inside of /etc are
configuration. Unit files, and symlinks in /usr are now valid for
installation instructions.

This patch is quite a rework of the whole install logic, and makes the
following addional changes:

- Adds a complete test "test-instal-root" that tests the install logic
  pretty comprehensively.

- Never uses canonicalize_file_name(), because that's incompatible with
  operation relative to a specific root directory.

- unit_file_get_state() is reworked to return a proper error, and
  returns the state in a call-by-ref parameter. This cleans up confusion
  between the enum type and errno-like errors.

- The new logic puts a limit on how long to follow unit file symlinks:
  it will do so only for 64 steps at max.

- The InstallContext object's fields are renamed to will_process and
  has_processed (will_install and has_installed) since they are also
  used for deinstallation and all kinds of other operations.

- The root directory is always verified before use.

- install.c is reordered to place the exported functions together.

- Stricter rules are followed when traversing symlinks: the unit suffix
  must say identical, and it's not allowed to link between regular units
  and templated units.

- Various modernizations

- The "invalid" unit file state has been renamed to "bad", in order to
  avoid confusion between UNIT_FILE_INVALID and
  _UNIT_FILE_STATE_INVALID. Given that the state should normally not be
  seen and is not documented this should not be a problematic change.
  The new name is now documented however.

Fixes #1375, #1718, #1706

src/basic/fs-util.c

@@ -233,6 +233,26 @@ int readlink_and_canonicalize(const char *p, char **r) {
         return 0;
 }
 
+int readlink_and_make_absolute_root(const char *root, const char *path, char **ret) {
+        _cleanup_free_ char *target = NULL, *t = NULL;
+        const char *full;
+        int r;
+
+        full = prefix_roota(root, path);
+        r = readlink_malloc(full, &target);
+        if (r < 0)
+                return r;
+
+        t = file_in_same_dir(path, target);
+        if (!t)
+                return -ENOMEM;
+
+        *ret = t;
+        t = NULL;
+
+        return 0;
+}
+
 int chmod_and_chown(const char *path, mode_t mode, uid_t uid, gid_t gid) {
         assert(path);
 

src/basic/fs-util.h

@@ -40,6 +40,7 @@ int readlink_malloc(const char *p, char **r);
 int readlink_value(const char *p, char **ret);
 int readlink_and_make_absolute(const char *p, char **r);
 int readlink_and_canonicalize(const char *p, char **r);
+int readlink_and_make_absolute_root(const char *root, const char *path, char **ret);
 
 int chmod_and_chown(const char *path, mode_t mode, uid_t uid, gid_t gid);
 int fchmod_and_fchown(int fd, mode_t mode, uid_t uid, gid_t gid);