Introduce mmap_file function

New in v2:

- As Tom pointed out, we don't need to keep the fd around after
  mmapping.  This simplifies things quite a bit, since we don't need a
  new class.  It's now just a function that returns a scoped_mmap.

We already have scoped_mmap, which is a thin RAII layer over mmap.  If
one simply wants to mmap an entire file for reading, it takes a bit of
boilerplate.  This patch introduces the mmap_file function to make this
easier.

gdb/ChangeLog:

	* Makefile.in (COMMON_SFILES): Add common/scoped_mmap.c.
	* common/scoped_mmap.c: New file.
	* common/scoped_mmap.h (destroy): New method.
	(~scoped_mmap, reset): Use destroy.
	(scoped_mmap): New move constructor.
	(mmap_file): New declaration.
	* unittests/scoped_mmap-selftests.c (test_normal,
	test_invalid_filename, run_tests): New functions.
	(_initialize_scoped_mmap_selftests): Register selftest.