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.
