implement CyaSSL_ERR_reason_error_string

this has several advantages:
- we can provide a replacement for openssl's ERR_reason_error_string,
  which makes porting simpler,
- code shrink due to removal of excessive strcpy call
- all error strings are const anyway so there's no point to force the
  user to supply storage for them and copying them around.
Merge branch 'const_errorstrings' of https://github.com/rofl0r/cyassl into errstr
update const error strings for newly added ones