xrootd: implement xrootd protocol

[sbinet]

Specs:

• http://xrootd.org/doc/dev45/XRdv310.htm
• http://xrootd.org/doc/dev49/XRdv400.htm

Status

• [x] kXR_admin
• [x] kXR_auth (see go-hep/hep#250)
  • [x] kXR_auth + host
  • [ ] kXR_auth + gsi
  • [x] kXR_auth + krb5
  • [ ] kXR_auth + pwd
  • [ ] kXR_auth + sss
  • [x] kXR_auth + unix
• [x] kXR_bind
• [x] kXR_chmod
• [x] kXR_close
• [x] kXR_decrypt
• [x] KXR_dirlist
• [x] KXR_endsess
• [ ] kXR_getfile
• [x] kXR_locate
• [x] kXR_login
• [x] kXR_mkdir
• [x] kXR_mv
• [x] kXR_open
• [x] kXR_ping
• [x] kXR_prepare
• [x] kXR_protocol
• [ ] kXR_putfile
• [x] kXR_query
• [x] kXR_read
• [ ] kXR_readv
• [x] kXR_rm
• [x] kXR_rmdir
• [ ] kXR_set
• [x] kXR_sigver
• [x] kXR_stat
• [x] kXR_statx
• [x] kXR_sync
• [x] kXR_truncate
• [x] kXR_write
• [ ] kXR_writev
• [x] kXR_verifyw

[sbinet]

@EgorMatirov this could be broken up into smaller pieces to swallow :) (e.g. the req/rep implementation for everything that pertains to "login" in "readonly", then those that need some basic authentication and basic permissions, etc...)

[sbinet]

@EgorMatirov the branch xrootd is just a first stab, giving an overview of what I think would be a good layout. open to discussion, of course.