tree-wide: use "hostname" spelling everywhere

It's not that I think that "hostname" is vastly superior to "host name". Quite
the opposite — the difference is small, and in some context the two-word version
does fit better. But in the tree, there are ~200 occurrences of the first, and
>1600 of the other, and consistent spelling is more important than any particular
spelling choice.

src/basic/hostname-util.c

@@ -21,7 +21,7 @@ bool hostname_is_set(void) {
         if (isempty(u.nodename))
                 return false;
 
-        /* This is the built-in kernel default host name */
+        /* This is the built-in kernel default hostname */
         if (streq(u.nodename, "(none)"))
                 return false;
 
@@ -77,7 +77,7 @@ bool valid_ldh_char(char c) {
 }
 
 /**
- * Check if s looks like a valid host name or FQDN. This does not do
+ * Check if s looks like a valid hostname or FQDN. This does not do
  * full DNS validation, but only checks if the name is composed of
  * allowed characters and the length is not above the maximum allowed
  * by Linux (c.f. dns_name_is_valid()). Trailing dot is allowed if