logging: support context-specific severity

Allow messages to have severity set to INFO or DEBUG depending on the
context in which they are made to allow logging important changes made
from chronyc or sourcefile, but not spam the system log if those changes
are normally expected (e.g. specified in the config).

cmdmon.c

@@ -1515,6 +1515,8 @@ read_from_cmd_socket(int sock_fd, int event, void *anything)
     }
 
     if (allowed) {
+      LOG_SetContext(LOGC_Command);
+
       switch(rx_command) {
         case REQ_NULL:
           /* Do nothing */
@@ -1762,6 +1764,8 @@ read_from_cmd_socket(int sock_fd, int event, void *anything)
           tx_message.status = htons(STT_FAILED);
           break;
       }
+
+      LOG_UnsetContext(LOGC_Command);
     } else {
       tx_message.status = htons(STT_UNAUTH);
     }