Prefix inet_net_ntop and sibling routines with "pg_" to ensure that
they aren't mistaken for C-library functions.  This fixes warnings
from cpluspluscheck on some platforms, and should help reduce reader
confusion everywhere, since our functions aren't exactly interchangeable
with the library versions (they may have different ideas about address
family codes).

This shouldn't be fixing any actual bugs, unless somebody's linker
is misbehaving, so no need to back-patch.

Discussion: https://postgr.es/m/20518.1559494394@sss.pgh.pa.us