log: minimize includes in log.h

log.h really should only include the bare minimum of other headers, as
it is really pulled into pretty much everything else and already in
itself one of the most basic pieces of code we have.

Let's hence drop inclusion of:

1. sd-id128.h because it's entirely unneeded in current log.h
2. errno.h, dito.
3. sys/signalfd.h which we can replace by a simple struct forward
   declaration
4. process-util.h which was needed for getpid_cached() which we now hide
   in a funciton log_emergency_level() instead, which nicely abstracts
   the details away.
5. sys/socket.h which was needed for struct iovec, but a simple struct
   forward declaration suffices for that too.

Ultimately this actually makes our source tree larger (since users of
the functionality above must now include it themselves, log.h won't do
that for them), but I think it helps to untangle our web of includes a
tiny bit.

(Background: I'd like to isolate the generic bits of src/basic/ enough
so that we can do a git submodule import into casync for it)

src/basic/log.h

@@ -20,18 +20,16 @@
   along with systemd; If not, see <http://www.gnu.org/licenses/>.
 ***/
 
-#include <errno.h>
 #include <stdarg.h>
 #include <stdbool.h>
 #include <stdlib.h>
-#include <sys/signalfd.h>
-#include <sys/socket.h>
 #include <syslog.h>
 
-#include "sd-id128.h"
-
 #include "macro.h"
-#include "process-util.h"
+
+/* Some structures we reference but don't want to pull in headers for */
+struct iovec;
+struct signalfd_siginfo;
 
 typedef enum LogRealm {
         LOG_REALM_SYSTEMD,
@@ -194,7 +192,7 @@ int log_struct_iovec_internal(
                 const char *file,
                 int line,
                 const char *func,
-                const struct iovec input_iovec[],
+                const struct iovec *input_iovec,
                 size_t n_input_iovec);
 
 /* This modifies the buffer passed! */
@@ -252,13 +250,15 @@ void log_assert_failed_return_realm(
 
 #define log_full(level, ...) log_full_errno((level), 0, __VA_ARGS__)
 
+int log_emergency_level(void);
+
 /* Normal logging */
 #define log_debug(...)     log_full(LOG_DEBUG,   __VA_ARGS__)
 #define log_info(...)      log_full(LOG_INFO,    __VA_ARGS__)
 #define log_notice(...)    log_full(LOG_NOTICE,  __VA_ARGS__)
 #define log_warning(...)   log_full(LOG_WARNING, __VA_ARGS__)
 #define log_error(...)     log_full(LOG_ERR,     __VA_ARGS__)
-#define log_emergency(...) log_full(getpid_cached() == 1 ? LOG_EMERG : LOG_ERR, __VA_ARGS__)
+#define log_emergency(...) log_full(log_emergency_level(), __VA_ARGS__)
 
 /* Logging triggered by an errno-like error */
 #define log_debug_errno(error, ...)     log_full_errno(LOG_DEBUG,   error, __VA_ARGS__)
@@ -266,7 +266,7 @@ void log_assert_failed_return_realm(
 #define log_notice_errno(error, ...)    log_full_errno(LOG_NOTICE,  error, __VA_ARGS__)
 #define log_warning_errno(error, ...)   log_full_errno(LOG_WARNING, error, __VA_ARGS__)
 #define log_error_errno(error, ...)     log_full_errno(LOG_ERR,     error, __VA_ARGS__)
-#define log_emergency_errno(error, ...) log_full_errno(getpid_cached() == 1 ? LOG_EMERG : LOG_ERR, error, __VA_ARGS__)
+#define log_emergency_errno(error, ...) log_full_errno(log_emergency_level(), error, __VA_ARGS__)
 
 #ifdef LOG_TRACE
 #  define log_trace(...) log_debug(__VA_ARGS__)