Submitted-By: Kent Vander Velden <graphix@iastate.edu>

functions are implimented as wrappers around getservent(), which means it's
up to getservent() to do all the work. The NIS support in getservent()
only allows it to scan through the services.byname map one entry at a
time until it finds the requested service name/port. This can be painfully
slow due to the overhead involved (lots and lots of successive RPCs).

To fix this, we allow getservbyname() and getservbyport() to signal
getservent() that if NIS is turned on (there's a '+' in /etc/services),
the usual yp_first()/yp_next() linear search should be abandoned and
yp_match() used instead. This causes getservent() to immediately
locate the requested entry instead of wasting time groping through the
whole map.

The downside is that this trick is accomplished by exporting a couple of
pointers from getservent.c which getservbyname.c and getservbyport.c can
preset in order to tell getservent() what to do. If all three functions
were in the same source module, then the extra cruft could be delcared
static to avoid poluting the global symbol space. Maybe they should be
combined anyway. For now I've settled on prepending lots of underscores.

an invalid pointer if a call to yp_first() fails.  Closes PR # 964,
and possibly # 952.

both call getservent() to do most of the work, so we only need to modify
this file to take care of everybody).

Note that there is only one NIS services map (services.byname) even
though there are getservbyname() and getservbyport() library functions.