Add GDALRegisterPlugin function to register a single plugin by name

[tbonfort]

This PR adds an exported CPLErr GDALRegisterPlugin(const char *name) function to
register a single plugin by name. Returns an error if either the plugin library
file was not found, or was found but did not contain a suitable plugin loading
function name, or if the plugin loading function CPLError'd.

Some of the plugin loading functionality has been copied over from
GDALDriverManager::AutoLoadDrivers() instead of being refactored into
a single function as I deemed the return-on-error behavior was too messy to
unify.

[coveralls]

coverage: 67.683% (-0.002%) from 67.685% when pulling 9f3b10c on tbonfort:load-plugin-by-name into f0095ee on OSGeo:master.