portfetch.tcl: Add option fetch.user_agent.
portfetch.tcl: Add option fetch.user_agent.

When MacPorts attempts to fetch from DropBox, it gets HTML instead
of the requested file. This is because MacPorts sets libcurl's User-Agent
header in curl.c to "MacPorts/MP_VERSION libcurl/LC_VERSION", which DropBox
responds to as a web brower. The problem was first reported in
https://trac.macports.org/ticket/53942 with regard to port
audio/generaluser-soundfont.

This PR adds an option `fetch.user_agent' which can be set in Portfile
as needed. For example, in audio/generaluser-soundfont/Portfile, adding:

  fetch.user_agent    curl/MacPorts/4.xx

allows fetch to work without a custom fetch block.  If `fetch.user_agent'
is not set, UserAgent as defined in curl.c is used instead.

In the particular case of DropBox, setting the User-Agent header to
"libcurl/..."  doesn't work as one might hope, otherwise swapping the
order of values in curl.c UserAgent variable would be suggested as an
alternative.

danchr/macports-base,neverpanic/macports-base,macports/macports-base,danchr/macports-base,neverpanic/macports-base,macports/macports-base