Merge branch 'fuzzing'

Adds support for fuzzing the certificate parser provided by the default
plugins (x509, pem, gmp etc.) on Google's OSS-Fuzz infrastructure (or
generally with libFuzzer). Fixes several issues that were found while
fuzzing these plugins.

When building the libraries monolithically and statically the
plugin constructors are now hard-coded in each library so the plugin
code is not removed by the linker because it thinks none of their symbols
are ever referenced.