srm: fix srmExtendFileLifeTime implementation

Addresses several issues:

- Improves error messages

- Uses standard compliant return codes

- Fixes seconds vs milliseconds bug in SURL lifetime implementation
  (which isn't supported by dCache, so an insignificant bug).

- Cleaned up the code (lots of public fields, unused fields, cut'n'paste
  comments that didn't belong, etc).

Target: trunk
Require-notes: yes
Require-book: no
Acked-by: Tigran Mkrtchyan <tigran.mkrtchyan@desy.de>
Patch: http://rb.dcache.org/r/6050/