Add documentation for gethostbyname_r, gethostbyname2_r and gethostbyaddr_r

Create proper MLINKs for the new functions.

PR: 249154
Reported by: asomers@

Before the change:

$ man gethostbyname_r
No manual entry for gethostbyname_r

After the change, the new documentation is accessible.

• mandoc -Tlint clean (useless .Tn macro and Xr named(8) reported)
• igor clean (deferring .Dd bump)
• aspell happy
• man ./gethostbyname.3 renders the page properly

Tested several things with this small program:

#include <err.h>
#include <netdb.h>
#include <stdio.h>

int
main()
{
        char *name = "localhost";
        char buflen = 100;
        char buffer[buflen];
        struct hostent he;
        struct hostent *result = NULL;
        int h_errnop, ret;

        if ((ret = gethostbyname_r(name, &he, buffer, buflen, &result, &h_errnop))) {
                errx(1, "no name associated with %s. result = %p. ret = %d",
                                name, result, ret);
        }

        printf("name associated with %s is %s\n", name, result->h_name);

        return 0;
}

Note that we don't have named(8) anymore in base, but I would remove that in
a different commit.

Also note the documentation for some getnet*_r functions is missing. I would
rather do this in small pieces.