make the stdb access functions a public API

[lennart]

we probably should add a new small lib "libavahi-stdb" which encapsulates the service type database access functions. Recommended functions:

avahi_stdb_lookup()
avahi_stdb_open()
avahi_stdb_close()
avahi_stdb_setstent()
avahi_stdb_getstent()

the last two functions should work like libc's getpwent(), setpwent().