Support "http.userAgent" and "http.extraHeader" from the git config

Support "http.userAgent" and "http.extraHeader" from the git config

Validate the extra headers and log but otherwise ignore invalid
headers. An empty http.extraHeader starts the list afresh.

The http.userAgent is restricted to printable 7-bit ASCII, other
characters are replaced by '.'.

Moves a support method from the ssh.apache bundle to HttpSupport in
the main JGit bundle.

Bug:541500
Change-Id: Id2d8df12914e2cdbd936ff00dc824d8f871bd580
Signed-off-by: James Wynn <james@jameswynn.com>
Signed-off-by: Thomas Wolf <thomas.wolf@paranor.ch>