gril: Fix warnings

Fix warnings to be able to use the same warning flags in bazaar branch
and upstream ofono.

* Add casts to be able to access gchar* as int32_t*
* Use GPOINTER_TO_INT and vice versa
* Fix types
src: Fix warnings

Fix warnings to be able to use the same warning flags in bazaar branch
and upstream ofono.

* stk.c not using right size for copying message
* Missing ssize_t definition
unit: Fix warnings

Fix warnings to be able to use the same warning flags in bazaar branch
and upstream ofono.

* Use g_test_trap_subprocess instead of deprecated g_test_trap_fork