cygport: allow setting prev:, curr:, and test: in autogenerated setup.hint

[Andrew Schulman <schulman dot andrew at epa dot gov>]

I have a package (socat) where I need to set the curr: and test: fields in
setup.hint.  I got tired of adding them to the autogenerated setup.hint files
after every build, so I wrote a patch for cygport to support specifying them in
the cygport file.  For example, setting

CURR=1.7.3.0-2
TEST=2.0.0-b8-1

in the cygport file will give

curr: 1.7.3.0-2
test: 2.0.0-b8-1

in setup.hint. The PREV, PKG_PREV, CURR, PKG_CURR, TEST, and PKG_TEST variables
are all supported.

The variable name TEST might be too generic, but I'm not aware of any other uses
for it, and grep -r '\<TEST\>' in the cygport source turns up none.  If
necessary it could be renamed to e.g. SETUP_TEST, but then probably CURR,
PKG_CURR etc. would all have to be renamed too.

The patch is in the setup.hint branch of
https://github.com/andrex-e-schulman/cygport.git.

Andrew