Similar to coverage report/xml/json, coverage run now has a -o option to
configure the file the SQLite coverage data will be stored in. The
default is still ".coverage", as previously.

Since data_file now needs to be specified in cmdline.py,
_DEFAULT_DATAFILE needs to be exposed (to avoid hacks like constructing
the argument dictionary manually). Rename it to non-undescore and create
an alias for it under the previous name (just in case someone used it
anyway).

-o for coverage run is not very useful if the commands that consume
coverage and generate reports/coverage (json/xml/html/combine ...) can't
take that file as input, so add a -c/--input-coverage option for these
commands.

Request by maintainer.