Make 'show width/height' display "unlimited" when capped for readline

When we cap the height/width sizes before passing to readline, tweak
the corresponding command variable to show "unlimited":

  (gdb) set height 0x8000
  (gdb) show height
  Number of lines gdb thinks are in a page is unlimited.

Instead of the current output:
  (gdb) set height 0x8000
  (gdb) show height
  Number of lines gdb thinks are in a page is 32768.

gdb/ChangeLog:
2019-02-27  Pedro Alves  <palves@redhat.com>

	* utils.c (set_screen_size): When we cap the height/width sizes,
	tweak the corresponding command variable to show "unlimited":

gdb/testsuite/ChangeLog:
2019-02-27  Pedro Alves  <palves@redhat.com>

	* gdb.base/page.exp: Add tests for "set/show width/height" with
	"infinite" values.