block signals during fork

there are several reasons for this. some of them are related to race
conditions that arise since fork is required to be async-signal-safe:
if fork or pthread_create is called from a signal handler after the
fork syscall has returned but before the subsequent userspace code has
finished, inconsistent state could result. also, there seem to be
kernel and/or strace bugs related to arrival of signals during fork,
at least on some versions, and simply blocking signals eliminates the
possibility of such bugs.

src/process/fork.c

@@ -13,7 +13,9 @@ weak_alias(dummy, __fork_handler);
 pid_t fork(void)
 {
 	pid_t ret;
+	sigset_t set;
 	__fork_handler(-1);
+	__block_all_sigs(&set);
 	ret = syscall(SYS_fork);
 	if (libc.main_thread && !ret) {
 		pthread_t self = __pthread_self();
@@ -22,6 +24,7 @@ pid_t fork(void)
 		libc.threads_minus_1 = 0;
 		libc.main_thread = self;
 	}
+	__restore_sigs(&set);
 	__fork_handler(!ret);
 	return ret;
 }