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.
After header and footer are read, pem_read_buffer() is able to determine
the length of input data used. This allows calling functions to skip
this PEM bit if an error occurs during its parsing.
(cherry picked from commit 9255e8300e)
- 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.
