Add a `curl version` Tcl command to Pextlib.

[RJVB]

This command can be useful for debugging connectivity issues when Pextlib becomes upgradeable via a dedicated port and uses a libcurl that is also installed through its own dedicated port.

The command returns a Tcl dict containing information about the libcurl version that Pextlib was built against as well as the libcurl it is currently running against; a simplified equivalent of curl --version. Obtaining a list of names of available feature is possible only since curl v7.87.0 so this information will probably be replaced with a simple feature mask in the Pextlib build installed by the MacPorts installer on most older Darwin versions.

See trac ticket #51516

[RJVB]

(FWIW, a version command for Pextlib itself might be useful too!)

[RJVB]

How does this proceed now, i.e. when will it be merged?

[neverpanic]

Now.

[RJVB]

Now.

:)