***
This commit contains further changes to user agent string in addition
to introduced in fd66e2fe29.
Now it will send installation prefix obtained by /proc/self/exe. It
should resist patching `apt` with sed or in other way. Reminding that
installation prefix provided in user agent http header is intended for
distinguishing Termux from derivatives.
Here are sample lines from my mirror logs showing usage of Termux repo
by third-party projects:
```
162.158.103.43 - - [21/Jan/2021:09:46:43 +0200] "GET /dists/stable/Release HTTP/1.1" 304 0 "-" "Termux-PKG/1.0 mirror-checker (termux-tools 0.104) Termux (pl.sviete.dom; install-prefix:/data/data/pl.sviete.dom/files/usr)"
162.158.210.8 - - [20/Jan/2021:06:24:54 +0200] "GET /dists/stable/InRelease HTTP/1.1" 404 146 "-" "Debian APT-HTTP/1.3 (2.1.15) Termux (vn.vhn.vsc; install-prefix:/data/data/vn.vhn.vsc/files/usr)"
162.158.210.142 - - [20/Jan/2021:06:24:54 +0200] "GET /dists/stable/Release.gpg HTTP/1.1" 304 0 "-" "Debian APT-HTTP/1.3 (2.1.15) Termux (vn.vhn.vsc; install-prefix:/data/data/vn.vhn.vsc/files/usr)"
```
***
CloudFlare firewall for termux.org and termux-mirror.ml hosts would be
adjusted to block all requests which do not have install-prefix set or
if the latter doesn't match one of the Termux.
