sd-journal: make prot_from_flags() static and rename

The function had just one caller and a name that didn't explain much.
Let's make it static and rename for clarity.

While at it, the only caller was not doing error handling correctly
— the function would potentially return a negative error value which
wasn't handled. In practice this couldn't happen, but let's remove
this ambiguity.

src/basic/util.c

@@ -23,24 +23,6 @@ int saved_argc = 0;
 char **saved_argv = NULL;
 static int saved_in_initrd = -1;
 
-int prot_from_flags(int flags) {
-
-        switch (flags & O_ACCMODE) {
-
-        case O_RDONLY:
-                return PROT_READ;
-
-        case O_WRONLY:
-                return PROT_WRITE;
-
-        case O_RDWR:
-                return PROT_READ|PROT_WRITE;
-
-        default:
-                return -EINVAL;
-        }
-}
-
 bool in_initrd(void) {
         int r;
         const char *e;

src/basic/util.h

@@ -19,8 +19,6 @@ static inline void save_argc_argv(int argc, char **argv) {
         saved_argv = argv;
 }
 
-int prot_from_flags(int flags) _const_;
-
 bool in_initrd(void);
 void in_initrd_force(bool value);