don't treat numeric port strings as servent records in getservby*()

don't treat numeric port strings as servent records in getservby*()

some applications use getservbyport to find port numbers that are not
assigned to a service; if getservbyport always succeeds with a numeric
string as the result, they fail to find any available ports.

POSIX doesn't seem to mandate the behavior one way or another. it
specifies an abstract service database, which an implementation could
define to include numeric port strings, but it makes more sense to
align behavior with traditional implementations.

based on patch by A. Wilcox. the original patch only changed
getservbyport[_r]. to maintain a consistent view of the "service
database", I have also modified getservbyname[_r] to exclude numeric
port strings.