logging: Introduce a print_filename flag for the logtarget

Introduce a print_filename attribute for each logtarget. Initialize it
with 1 to be backward compatible with earlier versions. The bit is taken
from an existint bitfield. There were at least six bits left of the byte.
diff --git a/include/osmocom/core/logging.h b/include/osmocom/core/logging.h
index 28f7549..655f7a4 100644
--- a/include/osmocom/core/logging.h
+++ b/include/osmocom/core/logging.h
@@ -134,6 +134,8 @@
 	unsigned int use_color:1;
 	/*! \brief should log messages be prefixed with a timestamp? */
 	unsigned int print_timestamp:1;
+	/*! \brief should log messages be prefixed with a filename? */
+	unsigned int print_filename:1;
 
 	/*! \brief the type of this log taget */
 	enum log_target_type type;
@@ -179,6 +181,7 @@
 
 void log_set_use_color(struct log_target *target, int);
 void log_set_print_timestamp(struct log_target *target, int);
+void log_set_print_filename(struct log_target *target, int);
 void log_set_log_level(struct log_target *target, int log_level);
 void log_parse_category_mask(struct log_target *target, const char* mask);
 int log_parse_level(const char *lvl);