plugin_manager: Load parser plugins by name

plugin_manager: Load parser plugins by name

A parser plugin is a plugin that has name "%s_parser.so", where "%s"
is the name of the parser. Implement a function that will try to find
a parser plugin by a name. If we can't find such plugin from the
global library path, try some other places as well, such as current
working directory and executable path.

In order to know the executable path, the plugin manager needs to be
initialized with the executable path. We are now doing it in the
beginning of the main function.

Signed-off-by: Timo Kokkonen <timo.t.kokkonen@iki.fi>