signal_init: Always unblock SIGCHLD. (#5243)

Inherited signal mask may block SIGCHLD, which causes libuv to hang at
epoll_wait.

Closes #5230

Helped-by: Nicolas Hillegeer <nicolas@hillegeer.com>
Helped-by: John Szakmeister <john@szakmeister.net>

Note: the #pragma gymnastics are a workaround for broken system headers on
macOS.

  signal.h:
      int  sigaddset(sigset_t *, int);
      #define     sigaddset(set, signo)   (*(set) |= __sigbits(signo), 0)
  sys/_types/_sigset.h:
      typedef __darwin_sigset_t            sigset_t;
  sys/_types.h:
      typedef __uint32_t   __darwin_sigset_t;      /* [???] signal set */

sigset_t is defined as unsigned int, but the sigaddset() ORs it with an int,
mixing the types.  So GCC generates a sign-conversion warning:

  sig.c:9:13: warning: implicit conversion changes signedness: 'int' to 'unsigned int' [-Wsign-conversion]
    (*(&s) |= __sigbits((sigset_t) 20), 0);
           ~~ ^~~~~~~~~~~~~~~~~~~~~~~~
  1 warning generated.

System headers are normally ignored when the compiler generates warnings:
https://gcc.gnu.org/onlinedocs/cpp/System-Headers.html

  > GCC gives code found in system headers special treatment. All warnings,
  > other than those generated by ‘#warning’ (see Diagnostics), are suppressed
  > while GCC is processing a system header. Macros defined in a system header
  > are immune to a few warnings wherever they are expanded. This immunity is
  > granted on an ad-hoc basis, when we find that a warning generates lots of
  > false positives because of code in macros defined in system headers.

Instead of the #pragma workaround, we could cast the sigset_t pointer:

    # if defined(__APPLE__)
      sigaddset((int *)&mask, SIGCHLD);
    # else
      sigaddset(&mask, SIGCHLD);
    # endif

but that could break if the headers are later fixed.

src/nvim/event/process.c

@@ -17,8 +17,7 @@
 # include "event/process.c.generated.h"
 #endif
 
-// {SIGNAL}_TIMEOUT is the time (in nanoseconds) that a process has to cleanly
-// exit before we send SIGNAL to it
+// Time (ns) for a process to exit cleanly before we send TERM/KILL.
 #define TERM_TIMEOUT 1000000000
 #define KILL_TIMEOUT (TERM_TIMEOUT * 2)