Handle -1 return from sysconf(_SC_GET{PW,GR}_R_SIZE_MAX)

POSIX says:

    Note that sysconf(_SC_GETGR_R_SIZE_MAX) may return -1 if there is no
    hard limit on the size of the buffer needed to store all the groups
    returned.

The example from POSIX uses a default buffer size of 1024 in that case.

Also, correct _SC_GETPW_R_SIZE_MAX to _SC_GETGR_R_SIZE_MAX in util_lookup_group.