getnetent(3): Add missing reentrant functions

Add documentation for gethostbyname_r, gethostbyname2_r and gethostbyaddr_r

Create proper MLINKs for the new functions.

PR: 249154
Reported by: asomers@
Approved by: manpages (0mp@), Pau Amma
Differential Revision: https://reviews.freebsd.org/D30469