buildserver: move code into main() method to always stop thread

By running the whole program in a main() function, it can be wrapped in
try/finally in order to stop the background display thread.  This is also
done in ./fdroid, its standard practice for Python CLI utilities.

makebuildserver

@@ -20,7 +20,7 @@ if not os.path.exists('makebuildserver') and not os.path.exists('buildserver'):
 
 
 boxfile = os.path.join(os.getcwd(), 'buildserver.box')
-serverdir = 'buildserver'
+tail = None
 
 parser = OptionParser()
 parser.add_option("-v", "--verbose", action="store_true", default=False,
@@ -293,6 +293,9 @@ def sha256_for_file(path):
         return s.hexdigest()
 
 
+def main():
+    global cachedir, cachefiles, config, tail
+
     for srcurl, shasum in cachefiles:
         filename = os.path.basename(srcurl)
         local_filename = os.path.join(cachedir, filename)
@@ -362,6 +365,7 @@ elif os.path.exists('/proc/cpuinfo'):
         if 'vmx' in contents or 'svm' in contents:
             config['hwvirtex'] = 'on'
 
+    serverdir = os.path.join(os.getcwd(), 'buildserver')
     logfilename = os.path.join(serverdir, 'up.log')
     if not os.path.exists(logfilename):
         open(logfilename, 'a').close()  # create blank file
@@ -446,5 +450,10 @@ v.box_add('buildserver', boxfile, force=True)
 
     os.remove(boxfile)
 
+
+if __name__ == '__main__':
+    try:
+        main()
+    finally:
         if tail is not None:
             tail.stop()