netxml-ups driver: alarms subscription bugfixes

1/ req. buffer reuse for resp. isn't a good idea, resp. body gets
mixed with resp. POST data
2/ CRLF should be used in HTTP comm. with the NMC (although LF alone
should be acceptable for tolerant HTTP message parsers (see RFC 2616,
section 19.3, 3rd paragraph), the one in NMC has problems with it)