tree-wide: check if errno is greater then zero

gcc is confused by the common idiom of
  return errno ? -errno : -ESOMETHING
and thinks a positive value may be returned. Replace this condition
with errno > 0 to help gcc and avoid many spurious warnings. I filed
a gcc rfe a long time ago, but it hard to say if it will ever be
implemented [1].

Both conventions were used in the codebase, this change makes things
more consistent. This is a follow up to bcb161b023.

[1] https://gcc.gnu.org/bugzilla/show_bug.cgi?id=61846

src/basic/cgroup-util.c

@@ -93,7 +93,7 @@ int cg_read_pid(FILE *f, pid_t *_pid) {
                 if (feof(f))
                         return 0;
 
-                return errno ? -errno : -EIO;
+                return errno > 0 ? -errno : -EIO;
         }
 
         if (ul <= 0)

src/basic/fileio.c

@@ -165,7 +165,7 @@ int read_one_line_file(const char *fn, char **line) {
         if (!fgets(t, sizeof(t), f)) {
 
                 if (ferror(f))
-                        return errno ? -errno : -EIO;
+                        return errno > 0 ? -errno : -EIO;
 
                 t[0] = 0;
         }
@@ -1064,7 +1064,7 @@ int fflush_and_check(FILE *f) {
         fflush(f);
 
         if (ferror(f))
-                return errno ? -errno : -EIO;
+                return errno > 0 ? -errno : -EIO;
 
         return 0;
 }

src/basic/glob-util.c

@@ -40,7 +40,7 @@ int glob_exists(const char *path) {
         if (k == GLOB_NOSPACE)
                 return -ENOMEM;
         if (k != 0)
-                return errno ? -errno : -EIO;
+                return errno > 0 ? -errno : -EIO;
 
         return !strv_isempty(g.gl_pathv);
 }
@@ -58,7 +58,7 @@ int glob_extend(char ***strv, const char *path) {
         if (k == GLOB_NOSPACE)
                 return -ENOMEM;
         if (k != 0)
-                return errno ? -errno : -EIO;
+                return errno > 0 ? -errno : -EIO;
         if (strv_isempty(g.gl_pathv))
                 return -ENOENT;
 

src/basic/in-addr-util.c

@@ -219,7 +219,7 @@ int in_addr_to_string(int family, const union in_addr_union *u, char **ret) {
         errno = 0;
         if (!inet_ntop(family, u, x, l)) {
                 free(x);
-                return errno ? -errno : -EINVAL;
+                return errno > 0 ? -errno : -EINVAL;
         }
 
         *ret = x;
@@ -236,7 +236,7 @@ int in_addr_from_string(int family, const char *s, union in_addr_union *ret) {
 
         errno = 0;
         if (inet_pton(family, s, ret) <= 0)
-                return errno ? -errno : -EINVAL;
+                return errno > 0 ? -errno : -EINVAL;
 
         return 0;
 }

src/basic/terminal-util.c

@@ -128,7 +128,7 @@ int read_one_char(FILE *f, char *ret, usec_t t, bool *need_nl) {
 
         errno = 0;
         if (!fgets(line, sizeof(line), f))
-                return errno ? -errno : -EIO;
+                return errno > 0 ? -errno : -EIO;
 
         truncate_nl(line);
 
@@ -212,7 +212,7 @@ int ask_string(char **ret, const char *text, ...) {
 
                 errno = 0;
                 if (!fgets(line, sizeof(line), stdin))
-                        return errno ? -errno : -EIO;
+                        return errno > 0 ? -errno : -EIO;
 
                 if (!endswith(line, "\n"))
                         putchar('\n');