libudev-util.c: Fix sysconf variable for util_lookup_group

Signed-off-by: Michael Forney <mforney@mforney.org>
libudev-util.c: 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.

Signed-off-by: Michael Forney <mforney@mforney.org>
Merge pull request #81 from michaelforney/default-buflen

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