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/virt.c

@@ -240,7 +240,7 @@ static int detect_vm_zvm(void) {
         _cleanup_free_ char *t = NULL;
         int r;
 
-        r = get_status_field("/proc/sysinfo", "VM00 Control Program:", &t);
+        r = get_proc_field("/proc/sysinfo", "VM00 Control Program", WHITESPACE, &t);
         if (r == -ENOENT)
                 return VIRTUALIZATION_NONE;
         if (r < 0)