io: retry fgets on EINTR (#7632)

The calls to `fgets` in `src/nvim/if_cscope.c` (and elsewhere) can show
communication errors to the user if a signal is delivered during its
system calls. For plugins that proxy subprocess output into cscope
requests, a `SIGCHLD` might *always* interfere with calls into `fgets`.

To see this in a debugger, put a breakpoint on `cs_reading_emsg` and
watch signals come in (with lldb, using `process handle --notify true
--pass true`).  Next, run a subcommand from neovim that calls through
cscope when it returns.  A tag picker plugin, like vim-picker and fzy,
with `cscopetag` and `cscopetagorder=0` set, reproduced this reliably.
The breakpoint will hit after a `SIGCHLD` is delivered, and `errno` will
be set to 4, `EINTR`.

The caller of `fgets` should retry when `NULL` is returned with `errno`
set to `EINTR`.

src/nvim/if_cscope.c

@@ -553,9 +553,15 @@ static int cs_cnt_matches(size_t idx)
 
   char *buf = xmalloc(CSREAD_BUFSIZE);
   for (;; ) {
+    errno = 0;
     if (!fgets(buf, CSREAD_BUFSIZE, csinfo[idx].fr_fp)) {
-      if (feof(csinfo[idx].fr_fp))
+      if (errno == EINTR) {
+        continue;
+      }
+
+      if (feof(csinfo[idx].fr_fp)) {
         errno = EIO;
+      }
 
       cs_reading_emsg(idx);
 
@@ -1381,9 +1387,16 @@ static char *cs_parse_results(size_t cnumber, char *buf, int bufsize,
   char *p;
   char *name;
 
+retry:
+  errno = 0;
   if (fgets(buf, bufsize, csinfo[cnumber].fr_fp) == NULL) {
-    if (feof(csinfo[cnumber].fr_fp))
+    if (errno == EINTR) {
+      goto retry;
+    }
+
+    if (feof(csinfo[cnumber].fr_fp)) {
       errno = EIO;
+    }
 
     cs_reading_emsg(cnumber);