log: add internal function that logs to syslog

Differences to the console logger:

* No coloring
* Always use module name, not filename:line-number

log.c

@@ -1,10 +1,12 @@
 #include "log.h"
 
+#include <stdlib.h>
 #include <stdio.h>
 #include <stdarg.h>
 #include <string.h>
 #include <stdbool.h>
 #include <errno.h>
+#include <assert.h>
 
 #include <syslog.h>
 
@@ -57,6 +59,50 @@ _log(enum log_class log_class, const char *module, const char *file, int lineno,
     printf("\n");
 }
 
+static void
+_sys_log(enum log_class log_class, const char *module, const char *file,
+         int lineno, const char *fmt, int sys_errno, va_list va)
+{
+    /* Map our log level to syslog's level */
+    int level = -1;
+    switch (log_class) {
+    case LOG_CLASS_ERROR:    level = LOG_ERR; break;
+    case LOG_CLASS_WARNING:  level = LOG_WARNING; break;
+    case LOG_CLASS_INFO:     level = LOG_INFO; break;
+    case LOG_CLASS_DEBUG:    level = LOG_DEBUG; break;
+    }
+
+    assert(level != -1);
+
+    const char *sys_err = sys_errno != 0 ? strerror(sys_errno) : NULL;
+
+    va_list va2;
+    va_copy(va2, va);
+
+    /* Calculate required size of buffer holding the entire log message */
+    int required_len = 0;
+    required_len += strlen(module) + 2;  /* "%s: " */
+    required_len += vsnprintf(NULL, 0, fmt, va2); va_end(va2);
+
+    if (sys_errno != 0)
+        required_len += strlen(sys_err) + 2; /* ": %s" */
+
+    /* Format the msg */
+    char *msg = malloc(required_len + 1);
+    int idx = 0;
+
+    idx += snprintf(&msg[idx], required_len + 1 - idx, "%s: ", module);
+    idx += vsnprintf(&msg[idx], required_len + 1 - idx, fmt, va);
+
+    if (sys_errno != 0) {
+        idx += snprintf(
+            &msg[idx], required_len + 1 - idx, ": %s", strerror(sys_errno));
+    }
+
+    syslog(level, "%s", msg);
+    free(msg);
+}
+
 void
 log_msg(enum log_class log_class, const char *module,
         const char *file, int lineno, const char *fmt, ...)