[SSSD] NSS: disable midpoint refresh for netgroups if ptask refresh is enabled

[Pavel Reichl]

Hello, 

please see attached patches.

patch #1 - disable midpoint refresh for netgroups if ptask refresh is
enabled

The rest of patches solves some minor problems that occurred while I
was working on 1st patch:

patch #2 - fixes sysdb_getnetgr to return ENOENT as is as is expected in
code
patch #3 - first check return value then access output parameter
patch #4 - some minor code style improvements, some lines over 80
columns, IMO strange indentation of string constants - feel free to
NACK.

Bye,

Pavel Reichl