gdbserver: enum gdb_signal casts

gdbserver: enum gdb_signal casts

This is code parsing RSP signal numbers, checking whether the numbers
are indeed valid/known GDB signals, and then converting to host signal
numbers.  I considered adding temporary enum gdb_signal variables
instead, but didn't really like the result.

gdb/gdbserver/ChangeLog:
2015-10-29  Pedro Alves  <palves@redhat.com>

	* server.c (handle_v_cont, process_serial_event): Add enum
	gdb_signal casts to signal parsing code.