smb: use samba authentication callbacks to authenticate url
gui: allow empty user/password on path access entry to delete old entry
samba: upgrade client to 4.1.13
smb: upgrade to non legacy libsmbclient interface
smb: eliminiate extra stat call on open to get file size

We now retreive file size when requested. This was an old sin that disallowed growing files, and causes overhead on open. It also used stat instead of fstat on existing file pointer.
utils: make source stat structure const for stat conversions
smb: eliminate unused m_url variable
smb: write custom smb.conf file to special://temp

This allow us to write it every start, and avoid messing with system
clients.
smb: allow using alternate maximum client version in negotiation
smb: allow full sized reads when smb protocol is allowed higher

This can improve performance if the reading client requests large chunks
fixup! smb: allow using alternate maximum client version in negotiation