Add support for unbuffered and zero sized Guile ports.

Add support for unbuffered and zero sized Guile ports.

gdb/ChangeLog

	* NEWS: Mention support for unbuffered Guile memory ports.
	* scm-ports.c (ioscm_memory_port): Update comments on end, size.
	(ioscm_lseek_address): Improve overflow calculation.
	(gdbscm_memory_port_fill_input): Add assert.
	(gdbscm_memory_port_write): Handle unbuffered ports.
	Handle large writes identical to Guile's fport_write.
	(gdbscm_memory_port_seek): Fix seeking past end check.
	(gdbscm_memory_port_close): Handle closing unbuffered port.
	(ioscm_parse_mode_bits): Recognize "0" for unbuffered ports.
	(ioscm_init_memory_port): Handle unbuffered ports.
	(ioscm_reinit_memory_port): Ditto.
	(ioscm_init_memory_port): Update size calculation.
	(gdbscm_open_memory): Support zero sized ports.

gdb/testsuite/ChangeLog

	* gdb.guile/scm-ports.c: New file.
	* gdb.guile/scm-ports.exp: Add memory port tests.

gdb/doc/ChangeLog

	* guile.texi (Memory Ports in Guile): Document support for unbuffered
	memory ports.