_getdeployconf assigns and exports the variable, it does not print the
value, so wrapping it in a command substitution ran it in a subshell and
always yielded an empty string. A MULTIDEPLOY_FILENAME saved by an
earlier run was therefore never restored on renewal and the hook
silently fell back to multideploy.yml. Call it the same way every other
deploy hook does.
Also treat a MULTIDEPLOY_FILENAME starting with '/' as an absolute path
instead of always resolving it under DOMAIN_PATH, so one deploy file can
live outside the certificate directory and be shared by all domains.
Names without a leading '/' keep resolving under DOMAIN_PATH as before.
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