implement --alias option to control fontdbutil

[phi-gamma]

Karl writes:

2. If you use argv[0] to change the behavior, please also make cmdline
   options to control it in the same way. It's trivial to do that, and
   that way, users can use an alias or their own trivial wrapper script to
   get the behavior they want with the name they want, which turns out to
   be surprisingly useful in all kinds of unforeseen situations.

My proposal is adding an option to the script, --alias, that can be used
to explicitly request fondbutil or mkluatexfontdb behavior. Example:

fontdbutil --alias=mkluatexfontdb

will update the database. Also,

fontdbutil --alias=mkluatexfontdb -h

will show the help message from mkluatexfontdb. Naturally, it should work
the other way round:

mkluatexfontdb --alias=fontdbutil

will update the database only if -u is set as well.

[phi-gamma]

I mistakenly sent a pull request to Khaled. Srsly github has a suboptimal interface.