networkd: add basic LLDP transmission support

Let's add some minimalistic LLDP sender support. The idea is that this is
either on or off, and all fields determined automatically rather than
configured explicitly.

src/basic/hostname-util.h

@@ -26,6 +26,7 @@
 bool hostname_is_set(void);
 
 char* gethostname_malloc(void);
+int gethostname_strict(char **ret);
 
 bool hostname_is_valid(const char *s, bool allow_trailing_dot) _pure_;
 char* hostname_cleanup(char *s);