Abstract the functionality of iterating over mapped memory
regions into a general purpose iterator function.
* procfs.c (iterate_over_mappings): New function, general purpose
iterator for memory sections.
(proc_iterate_over_mappings): Reimplement using iterate_over_mappings.
(solib_mappings_callback): New function, callback for above.
(info_proc_mappings): Reimpliment using iterate_over_mappings.
(info_mappings_callback): New function, callback for above.
* procfs.c (proc_set_watchpoint): Add cast to suppress warning.
