gps plugin: Coding style cleanup.

gps plugin: Coding style cleanup.

* Include <gps.h> and <pthread.h> unconditionally -- the code doesn't
  compile without these headers.
* Convert all data in cgps_data_t to gauge_t.
* Rename the "gps_data_read" variable to "data", protected by "data_lock".
* Handle errors and continue, allowing the following code to be outdented.
* Add "satellites-visible" in addition to "satellites-used".
* Remove newlines from log messages. Unify prefix to "gps plugin:".
* Unify function and type names to use the "cgps_" prefix.
* Don't check for NULL when calling free().