* regcache.c (struct regcache): Replace passthough_p with
readonly_p.
(regcache_xmalloc): Initialize readonly_p.
(build_regcache): Initialize readonly_p.
(regcache_save): New function.
(regcache_restore): New function.
(regcache_cpy): Re-implement using regcache_save and
regcache_restore.
(regcache_raw_read): Update.
(regcache_cooked_read): When a read-only cache, checked for cached
pseudo register values.
(regcache_raw_write): Assert that the cache is not read-only.
Remove code handling a non-passthrough cache.
* regcache.h (regcache_save): Declare.
(regcache_restore): Declare.