fileio: make get_status_field() more generic

All users of get_status_field() expect the field pattern to occur in
the beginning of a line, and the delimiter is ':'.

Hardcode this into the function, and also skip any whitespace before ':'
to support fields in files like /proc/cpuinfo. Add support for returning
the full field value (currently stops on first whitespace).

Rename the function so it's easier to ensure all callers switch to new
semantics.

src/basic/process-util.c

@@ -215,7 +215,7 @@ int get_process_capeff(pid_t pid, char **capeff) {
 
         p = procfs_file_alloca(pid, "status");
 
-        r = get_status_field(p, "\nCapEff:", capeff);
+        r = get_proc_field(p, "CapEff", WHITESPACE, capeff);
         if (r == -ENOENT)
                 return -ESRCH;