watch the database activity or TPS

This implementation is based on data fetched from pg_stat_database. It
is then analyzed to obtain some information like the TPS. This patch is
generic as it is based only on libpq, so all the platforms can use that.
monitor the disk I/O of the server

In this patch only Linux platform is supported by parsing
/proc/diskstats to get some disk info. A new generic API adapted to the
machine/m_* files has been added.
monitor the disk space of the server

We first query server to get PGDATA, then use statfs (on Linux) to get
some disk space information. Similar to the I/O case, a new generic API
for machine/m_* files has been added, and now only Linux is supported.