CLEANUP: ssl/cli: use a local context for "set ssl cafile"

CLEANUP: ssl/cli: use a local context for "set ssl cafile"

Just like for "set ssl cert", the command doesn't really need this
context which doesn't outlive the parsing function but it was there
for a purpose so it's maintained. Only 3 fields were used from the
appctx's ssl context: old_cafile_entry, new_cafile_entry, and path.
These ones were reinstantiated into a new "set_cafile_ctx" struct.
None of them could be dropped as they are still shared with other
commands.