specifier: use %q for pretty hostname

%R is already used in service manager specifier expansion (cgroup root),
hence use a different char, that was so far not used.

Follow-up for: 6ceb0a4094

src/shared/specifier.h

@@ -51,6 +51,7 @@ int specifier_var_tmp_dir(char specifier, const void *data, const char *root, co
  * %B: the OS build ID, according to /etc/os-release
  * %H: the hostname of the running system
  * %l: the short hostname of the running system
+ * %q: the 'pretty' hostname as per /etc/machine-info
  * %m: the machine ID of the running system
  * %M: the OS image ID, according to /etc/os-release
  * %o: the OS ID according to /etc/os-release
@@ -76,7 +77,7 @@ int specifier_var_tmp_dir(char specifier, const void *data, const char *root, co
         { 'B', specifier_os_build_id,     NULL }, \
         { 'H', specifier_host_name,       NULL }, \
         { 'l', specifier_short_host_name, NULL }, \
-        { 'R', specifier_pretty_host_name,NULL }, \
+        { 'q', specifier_pretty_host_name,NULL }, \
         { 'm', specifier_machine_id,      NULL }, \
         { 'M', specifier_os_image_id,     NULL }, \
         { 'o', specifier_os_id,           NULL }, \