Cleanup and improve source code comments

Global review, which includes the following changes to try to increase
consistency, update and improve the source code comments:

 - Spelling fixes.
 - Use American English forms.
 - Uppercase NULL, NUL and ASCII.
 - Use “Note: ” instead of the slightly cryptic “NB: ” form.
 - Write comments as proper sentences, including capitalizations and
   ending dots.
 - Move comments before the code, function or variable they refer to.
 - Move general function comments outside the body.
 - Convert function and variable description comments to doxygen.
 - Use one space before dot, exclamation and question marks.
 - Use ‘’ or “” instead of `' style quoting.
 - Remove author names from comments, already visible from “git blame”.
 - Mark strings for translators with “TRANSLATORS: ”.
 - Remove useless or outdated comments.
 - Fix comment indentation.
 - Standardize comment format:

   /* Short text comment. */

   /* Long text,
    * comment. */

   /*
    * Section text.
    */

0 files changed, 0 insertions, 0 deletions