gdbserver: Prevent stale/random values in register cache

When fetch_inferior_registers does not update all registers, this
patch assures that no stale register values remain in the register
cache.  On Linux platforms using the regsets interface, when one of
the ptrace calls used for fetching the register values returns an
error, this patch also avoids copying the random data returned from
ptrace into the register cache.  All unfetched registers are marked
"unavailable" instead.

gdb/gdbserver/ChangeLog:

	* linux-low.c (regsets_fetch_inferior_registers): Do not invoke
	the regset's store function when ptrace returned an error.
	* regcache.c (get_thread_regcache): Invalidate register cache
	before fetching inferior's registers.