gst-inspect: Use less -F -X everywhere as the pager

This emulates the default behaviour of git help pages, and also fixes
a bug on macOS where `less -F` doesn't display anything at all when
the output is shorter than one terminal screen.

Also moved the DEFAULT_PAGER define to after the includes, because
it's an unprefixed define.

Fixes gstreamer/gstreamer#330
3 jobs for fix-gst-inspect-macos in 2 minutes and 20 seconds (queued for 1 second)
Status Name Job ID Coverage
  Manifest
passed manifest #48187

00:00:20

 
  Build
passed build android arm64 api28 #48189

00:02:00

passed build fedora x86_64 #48188

00:01:33