[Dovecot] Runtime root directory

[Brad Robel-Forrest]

I'm considering modifying the Dovecot source to support a runtime
designation for the root directory instead of relying on the prefix
provided via configure. Once complete, it would be possible to move the
root directory around somewhat ad-hoc after installation without having to
rebuild the source. The primary driver for this is that I'd like to wrap a
binary build of Dovecot within an OSX application bundle. These bundles
can be placed anywhere the user desires, so Dovecot couldn't rely on a
particular build-defined root directory.

Before I get too far down the path, I'd like to know if there's a desire
for this type of enhancement. If so, I'd like to provide a diff that would
meet any other requirements for this change. There are two methods I was
considering. Feel free to suggest a better alternative as I'm not ecstatic
about either at the moment.

1. Find and change all uses of the prefix directory to instead prepend a
value obtained from the environment (say, DOVECOT_ROOT_DIR).

2. Find and change all uses of the prefix directory to instead prepend a
value obtained from the configuration file (say, a new setting called
"dovecot_root_dir").

In either case, if the value was not available or not a valid directory, I
would fall back to the build prefix. I would also make this functionality
optional via configure--defaulted "off".

  -brad