Avoid crashing FS after delivering a VM

When vm-storage-dir was set to an absolute path, FS would abort after
delivering a VM because we were trying to free(3) memory sitting in
the middle of a memory pool.

FS-5329
Set mod_voicemail to depend on mail-transport-agent

Without this dependency we might not end up with sendmail installed
which would be unnecessarily surprising when using mod_voicemail.