***
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.
Switching to a more "soft" solution which will be focused on apt only.
Apt will now send an application package name (com.termux) and prefix
(/data/data/com.termux/files/usr) as part of user-agent string. That
will allow to distinguish real Termux users from parties which download
our packages and binary-patching them for the custom prefix.
Note that this will disable user's possibility to set custom User-Agent
for apt.
In issue https://github.com/termux/termux-packages/issues/6160 I have found
that community repo "its-pointless.github.io" specifies us as maintainer
for its packages. This is NOT TRUE and potentially misleads people using
these packages.
Now TERMUX_PKG_MAINTAINER will contain a default value which is neutral
and not specify maintainer. So all packages now have to override it to
the correct value.
[skip ci]
%ci:no-build
Add a small stub which verifies whether /data/data/com.termux/files/usr is readable
into few important packages such like Bash or APT. If this check fails, e.g.
in case if binary was executed in program with different package name (non-Termux),
program will print warning and exit with error status immediately.
This should prevent use of prebuilt packages by third-party applications.
NOTE: for now it uses /data/data/com.termux/files/usr in reversed order to
prevent automatic "blind" patching with sed. Will replace with randomized
XOR-based variant in case if that measure won't be enough.
