make cov requires ctags-exuberant

[filmil]

@kpozin noticed that the magic flags --c-kinds=fp used by make cov to extract function signatures from Unicode source doesn't exist in ctags versions other than ctags-exuberant. (yay!)

We should, perhaps, make a hermetic version of make cov that doesn't make this assumption.

[kpozin]

As a workaround, universal-ctags, a maintained fork of ctags-exuberant, seems to be widely available in Debian/Ubuntu package repos. (Don't try to install the universal-ctags snap, though -- that won't work for make cov due to permission issues.)

[masatake]

If you use universal-ctags, I recommend you to use --kinds-c=fp instead of --c-kinds=fp.
Please, see https://docs.ctags.io/en/latest/man/ctags-incompatibilities.7.html#obsoleting-lang-kinds-option for more details.