main/nss-pam-ldapd: enable nss module

Fixes #7946 (closed) by enabling the nss module and addressing musl compatibility.

Defining NETDB_INTERNAL as -1 appears to be safe. It's set to -1 in glibc, musl doesn't use -1 for any other code, and the source doesn't seem to do any inequality operations on h_errnop. I've tested the changes with openldap and haven't noticed any issues.

glibc codes: https://github.com/bminor/glibc/blob/master/resolv/netdb.h#L72

musl codes: https://git.musl-libc.org/cgit/musl/tree/include/netdb.h#n123

musl implementation: https://git.musl-libc.org/cgit/musl/tree/src/network/gethostbyname2_r.c#n11

base test image: https://gitlab.o-connor.io/nick/ldap-alpine