stand/efihttp: Work around a bug in edk2 http instance reconfiguration

A bug in the EFI HTTP driver of TianoCore EDK2 causes memory
corruption when an http instance that uses tls is reconfigured,
leading to a crash.

Work around this by forcing a new http instance for each request
instead of reconfiguring the existing one.

The upstream bug report is https://bugzilla.tianocore.org/show_bug.cgi?id=1917

Submitted by: bcran
Reviewed By: imp, kevans, tsoome
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D21281