lib-storage: Plugin hook API changed to support per-user plugins within a process. --HG-- branch : HEAD

plugins: Added dependencies. The dependencies can only be checked if the dlopen() can successfully open the module, so use (apparently) non-portable ltlibrary linking to force the libraries we depend on to load with us. --HG-- branch : HEAD

Changed .h ifdef/defines to use <NAME>_H format. --HG-- branch : HEAD

Added mail_log plugin to log various mail operations. Currently it logs mail copies, deletions, expunges and mailbox deletions. --HG-- branch : HEAD