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/src/coding/gsm0503_mapping.c b/src/coding/gsm0503_mapping.c
index 15fd1b6..d414769 100644
--- a/src/coding/gsm0503_mapping.c
+++ b/src/coding/gsm0503_mapping.c
@@ -32,9 +32,8 @@
  *
  *  This module contains burst mapping routines as specified in 3GPP TS
  *  05.03 / 45.003.
- */
-
-/*! \file gsm0503_mapping.c */
+ *
+ * \file gsm0503_mapping.c */
 
 void gsm0503_xcch_burst_unmap(sbit_t *iB, const sbit_t *eB,
 	sbit_t *hl, sbit_t *hn)