Fix deletion of text holding `inhibit-read-only' properties

Fix deletion of text holding `inhibit-read-only' properties

* src/intervals.h (INTERVAL_VISIBLE_P): Split into ...
(INTERVAL_GENERALLY_WRITABLE_P, INTERVAL_EXPRESSLY_WRITABLE_P):
... two new macros.

* src/textprop.c (verify_interval_modification): If the buffer
is read only, verify not that there is only a single exempting
interval spanning the area of a multiple-character operation,
but that every intervening interval in such an operation exempts
it from write restrictions, either by providing a read-only
property that appears in Vinhibit_read_only, or by providing an
inhibit-read-only property.

* test/src/textprop-tests.el (textprop-interval-immutability):
New test.