{mysql,pg}{slower,stat} wrappers for dbslower/dbstat

Per @brendangregg 's [suggestion](https://github.com/iovisor/bcc/pull/970#issuecomment-279958323), move dbslower & dbstat to tools/lib, and make wrapper scripts that invoke the main script, making them more discoverable. E.g., for mysqlslower:

```bash
#!/bin/bash
lib=$(dirname)/lib
$lib/dbslower mysql "$@"
```

The man pages and examples files can be simple symlinks (shared between the tools):

```
tools/mysqlslower_example.txt -> tools/lib/dbslower_example.txt
tools/pgslower_example.txt -> tools/lib/dbslower_example.txt
tools/mysqlstat_example.txt -> tools/lib/dbstat_example.txt
tools/pgsqlstat_example.txt -> tools/lib/dbstat_example.txt
man/man8/mysqlslower.8 -> man/man8/dbslower.8
man/man8/pgslower.8 -> man/man8/dbslower.8
man/man8/mysqlstat.8 -> man/man8/dbstat.8
man/man8/pgstat.8 -> man/man8/dbstat.8
```