Preserve host interrupt handlers in evalScript

[carrerasdarren-cell]

Summary

• preserve an embedder's interrupt handler across std.evalScript() calls
• chain the embedder handler while the standard library's SIGINT handler is active
• add native regressions for timeouts both inside and after evalScript()

Fixes #1673.

Testing

• api-test in release and ASan/UBSan builds
• api-test with QJS_BUILD_LIBC both on and off
• run-test262 -c tests.conf (0/110 errors, 9 excluded)
• make jscheck ctest cxxtest
• supplied A/B reproducer: the evalScript() case hangs on untouched master and exits normally after this patch