Log a warning if user tries to change ShareDir in runtime

[bergie]

Until #8 and #9 are fixed, the Midgard2 extension only loads MgdSchemas from ShareDir at start-up time. This means that either:

• PHP is started with a specified config file (midgard.configuration_file INI setting), in which case the ShareDir from that is used
• PHP follows the MIDGARD_ENV_GLOBAL_SHAREDIR directive
• The global /usr/share/midgard2 directory is used

Now, if user later tries to open a Midgard connection with open(filename) or open_config(object) that has a different ShareDir set than what Midgard is already using (see the list above), we should log a warning about this. Otherwise users easily think their setting is working an may get into trouble.

[bergie]

The warning could be something like _Tried to change Midgard ShareDir from /original/path to /new/path in runtime. This has no effect. Use the MIDGARD_ENV_GLOBAL_SHAREDIR environment variable for this_.