More fixes based on the compiler directives -Wcast-qual -Wwrite-strings
-Wmissing-prototypes -Wmissing-declarations. Not everything with regards
to -Wcast-qual has been fixed as some have unwanted consequences for the
rest of the code.
All symmetric cipher algorithms and hash algorithms now include support
for a POLARSSL_XXX_ALT flag that prevents the definition of the
algorithm context structure and all 'core' functions.
(cherry picked from commit 4087c47043)
- The error codes have been remapped and combining error codes is now done with a PLUS instead of an OR as error codes used are negative.
- Descriptions to all error codes have been added.
- Generation script for error.c has been created to automatically generate error.c from the available error definitions in the headers.
