Add stub implementations of popen/pclose (#15013)

Fixes: emscripten-core/emsdk#890

Author: sbc100

src/library.js

@@ -183,6 +183,8 @@ LibraryManager.library = {
   },
   vfork: 'fork',
   posix_spawn: 'fork',
+  popen: 'fork',
+  pclose: 'fork',
 
   setgroups__deps: ['$setErrNo', 'sysconf'],
   setgroups__unimplemented: true,

tests/unistd/misc.c

@@ -129,6 +129,12 @@ int main() {
   printf("vfork: %d", vfork());
   printf(", errno: %d\n", errno);
   errno = 0;
+  printf("popen: %p", popen("/bin/ls", "r"));
+  printf(", errno: %d\n", errno);
+  errno = 0;
+  printf("pclose: %d", pclose(NULL));
+  printf(", errno: %d\n", errno);
+  errno = 0;
 
   printf("crypt: %s", crypt("foo", "bar"));
   printf(", errno: %d\n", errno);

tests/unistd/misc.out

@@ -27,6 +27,8 @@ fchown(good): 0, errno: 0
 fchown(bad): -1, errno: 8
 fork: -1, errno: 52
 vfork: -1, errno: 52
+popen: 0xffffffff, errno: 52
+pclose: -1, errno: 52
 crypt: ba4TuD1iozTxw, errno: 0
 encrypt, errno: 0
 getgid: 0, errno: 0

tools/system_libs.py

@@ -721,7 +721,7 @@ def get_files(self):
         'res_query.c', 'res_querydomain.c', 'gai_strerror.c',
         'proto.c', 'gethostbyaddr.c', 'gethostbyaddr_r.c', 'gethostbyname.c',
         'gethostbyname2_r.c', 'gethostbyname_r.c', 'gethostbyname2.c',
-        'alarm.c', 'syscall.c', 'popen.c',
+        'alarm.c', 'syscall.c', 'popen.c', 'pclose.c',
         'getgrouplist.c', 'initgroups.c', 'wordexp.c', 'timer_create.c',
         'faccessat.c',
         # 'process' exclusion