Move libldap/strdup.c to string.c.

libldap/string.c will hold various string.h replacements.
Removed liblutil/strdup.c
Moved ldap_pvt_strtok to string.c.
Moved prototype of ldap_pvt_strtok to <ac/string.h>.
Added #define strtok_r ldap_pvt_strtok (if strtok_r doesn't exist).