doxygen: unify use of \file across the board

Considering the various styles and implications found in the sources, edit
scores of files to follow the same API doc guidelines around the doxygen
grouping and the \file tag.

Many files now show a short description in the generated API doc that was so
far only available as C comment.

The guidelines and reasoning behind it is documented at
https://osmocom.org/projects/cellular-infrastructure/wiki/Guidelines_for_API_documentation

In some instances, remove file comments and add to the corresponding group
instead, to be shared among several files (e.g. bitvec).

Change-Id: Ifa70e77e90462b5eb2b0457c70fd25275910c72b
diff --git a/include/osmocom/core/utils.h b/include/osmocom/core/utils.h
index ce49cbe..855e653 100644
--- a/include/osmocom/core/utils.h
+++ b/include/osmocom/core/utils.h
@@ -5,9 +5,7 @@
 
 /*! \defgroup utils General-purpose utility functions
  *  @{
- */
-
-/*! \file utils.h */
+ * \file utils.h */
 
 /*! Determine number of elements in an array of static size */
 #define ARRAY_SIZE(x) (sizeof(x) / sizeof((x)[0]))