cpp: Return engine info for engine used by the context

lang/cpp/src/context.cpp (Context::engineInfo()): Return engine info

for protocol of context.

This change also fixes Context::setEngineFileName() and
Context::setEngineHomeDirectory() which overwrote home dir resp.
file name of the engine info for the protocol of context with the
corresponding value of the "first" engine info (i.e. engine info for
gpg) instead of keeping the current home dir resp. file name.

• GnuPG-bug-id: T5722