easy: rename struct monitor to socketmonitor

'struct monitor', introduced in 6cf8413e, already exists in an IRIX
header file (sys/mon.h) which gets included via various standard headers
by lib/easy.c

cc-1101 cc: ERROR File = ../../curl/lib/easy.c, Line = 458
"monitor" has already been declared in the current scope.

Reported-by: Tor Arntsen