[IOT-1475] Remove DER/PEM mixing in ParseChain

[IOT-1475] Remove DER/PEM mixing in ParseChain

Change cred resource to always provide own certificate as a PEM
string, and no longer require ParseChain to heuristically parse
a mixed bag of DER and PEM certs.

Also, go back to having ParseChain return an int rather than a size_t
which was changed during /W4 warning cleanup; it must be able to return
negative values to indicate errors.

Change-Id: Id36962ed580eb3bccc110aac0350349b05674ee7
Signed-off-by: Kevin Kane <kkane@microsoft.com>
Reviewed-on: https://gerrit.iotivity.org/gerrit/17835
Tested-by: jenkins-iotivity <jenkins@iotivity.org>
Reviewed-by: Alex Kelley <alexke@microsoft.com>
Reviewed-by: Greg Zaverucha <gregz@microsoft.com>