installer: copy smalloc.h on installation

fix #7485

Signed-off-by: Fedor Indutny <fedor@indutny.com>
doc: correct check for failed child_process.spawn

Since 0.10.x, exec failures are no longer signaled through stderr,
but rather as error events.

Signed-off-by: Fedor Indutny <fedor@indutny.com>
Merge remote-tracking branch 'upstream/master'