Rewrite scanner logic

Initially, the scanner used libmagic which used magic numbers in the file's
content to detect what kind of file it appears to be. Since that library isn't
available on all systems, we added support for two other libraries, mimetypes
amongst them.

The issue with mimetypes is that it only uses the file's extension, not its
actual content. So this ends in variable behaviour depending on what system
you're using fdroidserver on. For example, an executable binary without
extension would be ignored if mimetypes was being used.

We now drop all libraries - mimetypes too as it depends on the system's
mime.types file - and instead check extensions ourselves. On top of that, do
a simple binary content check to find binary executables that don't have an
extension.

The new in-house code without any dependencies doesn't add any new checks, so
no builds should break. The current checks still work:

 % fdroid scanner app.openconnect:1029
[...]
Found executable binary at assets/raw/armeabi/curl
Found executable binary at assets/raw/mips/curl
Found executable binary at assets/raw/x86/curl
Found JAR file at lib/XposedBridgeApi-54.jar
Found JAR file at libs/acra-4.5.0.jar
Found JAR file at libs/openconnect-wrapper.jar
Found JAR file at libs/stoken-wrapper.jar
Found shared library at libs/armeabi/libopenconnect.so
Found shared library at libs/armeabi/libstoken.so
Found shared library at libs/mips/libopenconnect.so
Found shared library at libs/mips/libstoken.so
Found shared library at libs/x86/libopenconnect.so
Found shared library at libs/x86/libstoken.so

docs/fdroid.texi

@@ -122,8 +122,6 @@ vagrant-cachier plugin (unpackaged): `vagrant plugin install vagrant-cachier`
 Paramiko (debian package python-paramiko)
 @item
 Imaging (debian package python-imaging)
-@item
-Magic (debian package python-magic)
 @end itemize
 
 On the other hand, if you want to build the apps directly on your system