reset context before makecontext call, check

check return value of swapcontext
replace 1 with EXIT_FAILURE in all err calls
add in_isr sighndl safeguard
mitigate against signals when signals are off
mask all signals in signal handler
rework sigmasks

only mask signals actively being used
always enable shutdown
Merge pull request #348 from LudwigOrtmann/native_fixup

Native fixup