gitfaq: add documentation on proxies

gitfaq: add documentation on proxies

Many corporate environments and local systems have proxies in use.  Note
the situations in which proxies can be used and how to configure them.
At the same time, note what standards a proxy must follow to work with
Git.  Explicitly call out certain classes that are known to routinely
have problems reported various places online, including in the Git for
Windows issue tracker and on Stack Overflow, and recommend against the
use of such software, noting that they are associated with myriad
security problems (including, for example, breaking sandboxing and image
integrity[0], and, for TLS middleboxes, the use of insecure protocols
and ciphers and lack of certificate verification[1]). Don't mention the
specific nature of these security problems in the FAQ entry because they
are extremely numerous and varied and we wish to keep the FAQ entry
relatively brief.

[0] https://issues.chromium.org/issues/40285192
[1] https://faculty.cc.gatech.edu/~mbailey/publications/ndss17_interception.pdf

Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>