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/ex_cmds2.c

@@ -3199,8 +3199,14 @@ static char_u *get_one_sourceline(struct source_cookie *sp)
     ga_grow(&ga, 120);
     buf = (char_u *)ga.ga_data;
 
+retry:
+    errno = 0;
     if (fgets((char *)buf + ga.ga_len, ga.ga_maxlen - ga.ga_len,
               sp->fp) == NULL) {
+      if (errno == EINTR) {
+        goto retry;
+      }
+
       break;
     }
     len = ga.ga_len + (int)STRLEN(buf + ga.ga_len);