os: add a convenient way to ignore certain system signals

[shove70]

This PR:

1. Added a convenient way to ignore certain system signals. It handles things differently depending on whether the current thread is the main thread or the background thread.
2. It's also part of improving the robustness of http.get() and can be useful in net applications.
3. Calling it is as simple as in go, in go: signal.Ignore(syscall.SIGPIPE, syscall.SIGILL, syscall.SIGURG)

import os
os.signal_ignore(.pipe, .urg)

[medvednikov]

@shove70 somehow it broke v self on macos:

$ cc  -std=gnu99 -w -o v1.exe ./vc/v.c -lm -lpthread
./vc/v.c:37939:31: error: scalar initializer cannot be empty
                sigset_t mask1 = ((sigset_t){EMPTY_STRUCT_INITIALIZATION});
                                            ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1 error generated.

had to revert here and in vc.git

[shove70]

@shove70 somehow it broke v self on macos:

$ cc  -std=gnu99 -w -o v1.exe ./vc/v.c -lm -lpthread
./vc/v.c:37939:31: error: scalar initializer cannot be empty
                sigset_t mask1 = ((sigset_t){EMPTY_STRUCT_INITIALIZATION});
                                            ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1 error generated.

had to revert here and in vc.git

Oh, I'm so sorry!

[medvednikov]

No worries, just github actions acting weird. It should've caught it.