libxl: event system: pass gc, not just ctx, to internal sigchld manipulators

libxl: event system: pass gc, not just ctx, to internal sigchld manipulators

We are going to want the gc for libxl__ev_fd_register.

No functional change in this patch.  Simply change the argument types,
and the actual arguments from ctx to gc.  Inside these functions, use
CTX (the macro which uses gc) rather than the old formal parameter
ctx.

Signed-off-by: Ian Jackson <Ian.Jackson@eu.citrix.com>
Cc: Ian Campbell <ian.campbell@citrix.com>