Use -Wl,--as-needed -Wl,--no-undefined GNU ld flags for non-plugins.

Added dovecot.m4 file that external plugins can use. Based on patch by Dennis Schridde. --HG-- branch : HEAD

OSX: Don't try add plugin dependencies. It just breaks plugin loading. --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

Get rid of imap/, pop3/, lda/ and sql/ module subdirs. It's all in lib/dovecot/ now. --HG-- branch : HEAD

Previous changes broke plugin symlink creation in make install. --HG-- branch : HEAD

Use dynamic module suffix provided by libtool rather than hardcoded ".so". --HG-- branch : HEAD

Better type safety to module_contexts arrays. Already fixed some bugs. --HG-- branch : HEAD

Changed the plugin filename numbering (ie. the ordering in which the plugins are loaded). This doesn't change anything for current plugins, but future plugins will need this change so they can get themselves loaded between specific plugins. --HG-- branch : HEAD

mkdir_p doesn't accept multiple directories in some systems, so do it one at a time. This also cleans up the code. --HG-- branch : HEAD

Load plugins in sorted order. Prefixed plugins with nn_ set their order. --HG-- branch : HEAD

Added missing header files. Fixed plugin install hooks to use DESTDIR. --HG-- branch : HEAD

Added beginnings of plugin infrastructure. TODO: These could be optionally compiled into binaries with some configure options. Added quota plugin and a new trash plugin. Not very well tested. --HG-- branch : HEAD