fileio: modernize get_proc_field()

- Drop effectively unused "terminator" param, imply whitespace
- Make ret param optional
- Return ENODATA if the requested key is not found, rather than
  ENOENT
- Turn ENOENT -> ENOSYS if /proc/ is not mounted
- Don't skip whitespaces before ':', nothing needs this handling
  anyways
- Remove the special treatment for all "0"s. We don't actually
  use this for capabilities given pidref_get_capability() exists
- Switch away from read_full_virtual_file() - files using "field"
  scheme under /proc/ seem all to be "seq_file"s (refer to
  da65941c3e for details on file types)

src/basic/fileio.h

@@ -98,7 +98,7 @@ int verify_file_at(int dir_fd, const char *fn, const char *blob, bool accept_ext
 
 int script_get_shebang_interpreter(const char *path, char **ret);
 
-int get_proc_field(const char *filename, const char *pattern, const char *terminator, char **field);
+int get_proc_field(const char *path, const char *key, char **ret);
 
 DIR* xopendirat(int dir_fd, const char *name, int flags);