mobicore: do not use GNU old-style field designators

Avoiding the use of gnu extensions improves code portability

Change-Id: I0523b67dc649633f5e019d3e9b8af974251b9974
Switch from getdtablesize(3) to sysconf(3).

Change-Id: I4741ea034fdadf1c6cd07c36aee3156f2415fae2