rilmodem: Fix GPRS attach/detach logic ( LP: 1234434 ).

When the primary context was shutdown, the active_ctx_cid was
being reset before rilmodem notified the core that the context
had been deactified.  This meant an invalid (-1) cid was passed
to the core, and thus the context wasn't being cleaned up properly,
which in turn could cause the attach code to quit responding to
subsequent attaches ( either from DBus requests, or DATA
registration changes.
rilmodem/gprs-context: re-factored code to make all context ids gints.

The rilmodem gprs-context code internally stored context ids
as guints, and used -1 to indicate IDLE/disconnected state.
As rild and the core ofono code only used gints, this led
to casting issues, and also was confused the code, as the
gprs_context_data struct also includes a status field which
more clearly be used to check state.
Merge pull request #8 from tonyespy/gprs-lp1234434

Gprs lp1234434
Update of README to clarify Ubuntu-specific usage of this repository.
Add github repository to README and further clarify description.
One final tweak to README wording.
Merge pull request #12 from tonyespy/update-readme

Update of README to clarify Ubuntu-specific usage of this repository.
[gril] Fix compilation issues

Convert serial_no and request id gint instead of guint. Add missing
library dependency to unit tests. Remove initializing of struct
member twice. Move debug message after block's variables are
initialized. Remove unused ril_reply struct. Change also some other
parcel related fields to gint for consistency because the ril uses
signed int internally for parcel. Change cid to gint.
[rilmodem] Fix compilation issues

Replace loop counter with guint because num_apps in sim_status
also uses guint.
[plugins/ril] Fix compilation issues.

Change g_ril_send return to be handled as int. Add used header files
that provide used symbols. Remove unused variables and functions.
Merge pull request #14 from miksa/pull-request

Fix compilation issues when -Werror is used.
Merge remote-tracking branch 'rilmodem/master' into sim-refactor