Ignore SIGURG at Start command

[vr009]

This patch fixes the problem, which was occurring when the watchdog
process received a signal SIGURG from go runtime[1][2] and passed it
to the forked process before the exec call. Fixed by adding a call of
the Ignore function. Receiving this signal is unexpected, cause tt
doesn't work with sockets at all.

Also this patch fixes the signals handling strategy. Now there is only
one handling loop for all signals. Added two fields for controlling
a state of the instance being watched and a mutex for synchronizing
the goroutines changing them.

[1] - https://go.googlesource.com/proposal/+/master/design/24543-non-cooperative-preemption.md
[2] - golang/go#37942

Closes #325

[LeonidVas]

Hi! Thank you for the patchset.
It's the review of the first commit.

• Add Part of #... to the commit message.
• Strange that it was not necessary to make changes to the Watchdog tests (Are you sure you don't need them?)
• I don't like the fact that you have to start the signal processing from the outside. Would it make sense to add a callback that would be set externally and called by watchdog after the start of the application or something like that?

[vr009]

Strange that it was not necessary to make changes to the Watchdog tests (Are you sure you don't need them?).

I don't like the fact that you have to start the signal processing from the outside. Would it make sense to add a callback that would be set externally and called by watchdog after the start of the application or something like that?

Check the idea, please.