fix #17749 ignore SIGPIPE signals, fix nim CI #17748

[timotheecour]

• fix osproc: writes can crash process with SIGPIPE, instead of raising IOError on EPIPE #17749
• fix nim CI fails on i386 SIGPIPE nimsuggest/tester #17748 although it's not 100% clear why, but the change in this PR is good anyways
  (install issues for i386 seemed intermittent refs CI fails on azure 32 bit linux Linux_i386: sudo apt-fast install fails #17325)
• posix is now consistent with windows in that regard (raising exception instead of raising signal that aborts process); however as noted in PR, windows raises OSError, posix raises IOError; this can be fixed in future work

future work

• add a lib/std/private/signals module to refactor posix signal specific code and avoid duplicating logic in several places (ansi_c, system/excpt, posix, segfaults, as well as simplify code eg: stdlib/tssl, rawsockets, blockSigpipe in lib/pure/net.nim
• use sigaction instead of blockSigpipe, sigaction is deprecated and comes with many caveats, in particular for multithreaded programs
  https://stackoverflow.com/questions/231912/what-is-the-difference-between-sigaction-and-signal

Use sigaction() unless you've got very compelling reasons not to do so.

• https://stackoverflow.com/questions/108183/how-to-prevent-sigpipes-or-handle-them-properly etc
• https://stackoverflow.com/questions/7774569/using-signals-and-sigpipe

• use the approach suggested in http://www.microhowto.info/howto/ignore_sigpipe_without_affecting_other_threads_in_a_process.html (maybe it's blockSigpipe in lib/pure/net.nim) instead of c_signal(SIGPIPE, SIG_IGN), but this would require quite a bit of refactoring and should be done when we migrate signal to sigaction
• windows should raise IOError on pipe error, consistent with posix

links

refs #9867 (comment)

Also I'd recommend that everyone running their Nim program as a SSL server should install SIG_IGN as the SIGPIPE handler (actually we might want to do this in the stdlib). SIGPIPE is a pretty old signal and is largely obsoleted by EPIPE that's returned from any failed send()/write() attempt on a closed socket/pipe (we don't get to handle this one because SIGPIPE got in the way).