Add new config parser

The current config parser, parses the ini file and pulls out the values
specified by the struct config_section passed to parse_config_file() and
then throw the rest away.  This means that every place we want to get
info out of the ini file, we have to parse the whole thing again.  It's not
a big overhead, but it's also not a convenient API.

This patch adds a parser that parses the ini file to a data structure and
puts that in weston_compositor->config along with API to query comfig
keys from the data structure.  The old parser is still available, but
we'll transition to the new approach over the next few commits.