Add OS and arch to HTTP user-agent string

[hvr]

This makes the HTTP user-agent field look like so:

User-Agent: cabal-install/1.21.0.0 (linux; x86_64)

This allows us to gather more statistics about our user-base as
with the current user-agent string we can only do a rough break-down by
cabal-install version numbers.

[hvr]

Fyi, here's a quick'n'dirty statistical break-down from over the last two weeks extracted from the hackage.h.o logs (the IP address was taken into account as a heuristic to avoid counting the same user multiple times when accessing hackage with the same version):

visitors	cabal-install version
2	0.6.0
2	0.6.2
41	0.8.0
19	0.8.2
162	0.10.2
2	0.10.3
1849	0.14.0
11	0.17.0
12	1.16.0
67	1.16.0.1
6466	1.16.0.2
18	1.17.0
27	1.18.0
179	1.18.0.1
2538	1.18.0.2
1	1.18.0.2.0
1	1.18.0.2.1
3658	1.18.0.3
9	1.19.0
19	1.19.1
363	1.19.2
49	1.20.0
3320	1.20.0.0
129	1.21.0.0