always use a user-agent constant
always use a user-agent constant

Some providers block connections with missing user-agents. Also
user-agents are useful for analyzing requests. I've added a USER_AGENT
constant that is used in cURL and in file_get_contents.

michaelletzgus/nextcloud-server,michaelletzgus/nextcloud-server,andreas-p/nextcloud-server,pollopolea/core,pollopolea/core,endsguy/server,lrytz/core,owncloud/core,andreas-p/nextcloud-server,bluelml/core,owncloud/core,Ardinis/server,owncloud/core,IljaN/core,xx621998xx/server,michaelletzgus/nextcloud-server,cernbox/core,cernbox/core,endsguy/server,owncloud/core,xx621998xx/server,jbicha/server,xx621998xx/server,bluelml/core,bluelml/core,Ardinis/server,xx621998xx/server,andreas-p/nextcloud-server,whitekiba/server,jbicha/server,pollopolea/core,lrytz/core,pixelipo/server,sharidas/core,pmattern/server,whitekiba/server,endsguy/server,jbicha/server,pixelipo/server,pmattern/server,whitekiba/server,sharidas/core,pmattern/server,Ardinis/server,jbicha/server,nextcloud/server,bluelml/core,bluelml/core,endsguy/server,phil-davis/core,andreas-p/nextcloud-server,sharidas/core,Ardinis/server,cernbox/core,pmattern/server,lrytz/core,cernbox/core,lrytz/core,IljaN/core,whitekiba/server,pollopolea/core,nextcloud/server,pixelipo/server,pixelipo/server,lrytz/core,Ardinis/server,cernbox/core,andreas-p/nextcloud-server,IljaN/core,endsguy/server,owncloud/core,michaelletzgus/nextcloud-server,pollopolea/core,IljaN/core,xx621998xx/server,nextcloud/server,whitekiba/server,IljaN/core,sharidas/core,sharidas/core,jbicha/server,pixelipo/server,pmattern/server,nextcloud/server