Implement an `inhibit-read-only' text property

Implement an `inhibit-read-only' text property

* doc/lispref/text.texi (Special Properties): Mention `inhibit-read-only'.

* src/buffer.c (Fbarf_if_buffer_read_only): Don't raise an error if
the text at POSITION (new optional argument) has the
`inhibit-read-only' text property set.

* src/callint.c (Fcall_interactively): Pass in nil as argument to
Fbarf_if_buffer_read_only.

* src/fileio.c (Finsert_file_contents): Ditto.

* src/insdel.c (prepare_to_modify_buffer_1): Pass start region in.

* src/intervals.h (INTERVAL_WRITABLE_P): Check the `inhibit-read-only'
text property.

* src/textprop.c (verify_interval_modification): Check buffer
readedness after the last interval.