Use the correct Content-Type header for CAPTCHA

Some security inspection appliances and load balancers don't appreciate
something in the HTTP headers that is not a valid HTTP header. Furthermore,
the browser needs the Content-Type header to identify that the image is not
the PHP default of text/html
Don't send emails for non-existent table

When an admin logs in to upgrade to 1.7.1 and further from a version
pervious to 1.7.1, the system will attempt to clear password reset tokens
from the config table, which hasn't been upgraded yet to the namespaced
version from 1.7.1
Merge branch 'issue/squelch-pre-upgrade' into hotfix/171
ROOT_DIR should be used to identify filesystem paths

Historically, ROOT_PATH and ROOT_DIR contained the same value; however,
ROOT_PATH now points to the URL path where osTicket is installed, whereas
ROOT_DIR points to the file system location where osTicket is installed.
Merge branch 'issue/root-dir' into hotfix/171
Include setup/tips.html in download package
Merge branch 'issue/package-tips-html' into hotfix/171
Handle attachment filenames better

Previously, filenames saved in the database had the spaces changed for
underbars; however, other characters (such as commas and non-ascii
characters) presented issues with user agents downloading the attachments.

This patch handles the filename encoding for two special cases -- internet
explorer and safari, and provides the semi-standard RFC5987 method of
encoding the filename for the remaining browsers.

Attachments are no longer forced to be downloaded. It is up to the browser
to decide if the attachment should be shown in the browser or downloaded.

This patch also fixes a slight bug in the caching mechanism for downloads
concerning the last-modified time. The date sent to the browser was not
properly converted to GMT time, although the server claimed that it was.
Merge branch 'issue/filename-commas' into hotfix/171
Define ROOT_PATH in chrooted environments too
Merge branch 'issue/root-path' into hotfix/171
Merge branch 'hotfix/171'