tools/xenstore: try to get minimum thread stack size for watch thread

tools/xenstore: try to get minimum thread stack size for watch thread

When creating a pthread in xs_watch() try to get the minimal needed
size of the thread from glibc instead of using a constant. This avoids
problems when the library is used in programs with large per-thread
memory.

Use dlsym() to get the pointer to __pthread_get_minstack() in order to
avoid linkage problems and fall back to the current constant size if
not found.

Signed-off-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Wei Liu <wei.liu2@citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>
Tested-by: Jim Fehlig <jfehlig@suse.com>