Dumping certs, keys, and reqs as text

[rick_dean]

The attached patch implements dumping as text (like the -text option of
openssl programs). The interface extends dump_certificate(), dump_privatekey(),
and dump_certificate_request() with a new FILETYPE_TEXT.
The patch also adds test cases for dump_certificate(),
dump_privatekey(), dump_certificate_request(), load_certificate(),
load_privatekey(), and load_certificate_request() for
all FILETYPES.

The patch applies to revision 95 of branch ext-context.

[Jean-Paul Calderone (exarkun)]

Merged the dump_text branch.