RFE: add a proper man page for poptrc

[pmatilai]

Popt is supposed to be user customizable, and for that users shouldn't have to wade through API documentation.
Split the alias definition syntax out of popt(3) man page to a poptrc(5) manual.

[royagilmore]

I don't know if this should go here, or in a new issue, but --POPTdesc and --POPTargs are not documented in popt(3) man page, or anywhere else other than being used in src/poptconfig.c, tests/test-poptrc, and tests/test-poptrc.in, but there are no comments in the files about their use. An end user shouldn't have to read and understand the source code to edit a configuration file.