getpwnam_r()/getgrnam_r() returns ERANGE for some systems

getpwnam_r()/getgrnam_r() returns ERANGE for some systems

On our system the expected buffer length is 256. This means calls to
getpwnam_r()/getgrnam_r() return ERANGE error and corosync fails to startup.
These 2 functions return ERANGE when insufficient buffer space is supplied.
Judging by the man page for getpwnam_r, the correct way to determine the
buffersize on any given system is to use sysconf().

Signed-off-by: Tim Beale <tim.beale@alliedtelesis.co.nz>
Reviewed-by: Steven Dake <sdake@redhat.com>