main/mandoc: detect at run-time if the pager supports -T
This is useful to ensure that we support tag files with mandoc if main/less is installed (which it will be on most systems). Tag file support in mandoc is extremely useful as it allows one to, for example, directly lookup options in a man page.
This patch is (hopefully) upstreamable.
Alternative solutions:
- Implement the -T flag in BusyBox less
- Add a runtime dependency on main/less to mandoc
- Make BusyBox less ignore the -T flag