Make patches relative to where tarballs are extracted

Ever since an old patch version, it has (for reasonable security
reasons) not supported patched with ../ in the filename.
Many of our patches have been relying on this behaviour being OK,
because we start off with an ancient patch version that didn't perform
such checks. As soon as we need this behaviour after we build a newer
patch though, we will have problems.

So, let's change the policy.
Patches are relative to where tarballs are extracted, rather than the
"working directory" - e.g. have patches for `coreutils-9.4/src/cp.c`
instead of `src/cp.c`.
Keeping this consistent has a few implications;
- patches are applied from the build/ directory in bash era now, with
  `-p0`
- when patches are manually applied in the bash era, use `-p` as
  required, usually `-p1`
- in kaem era where patches are always manually applied, `-p1` is used

steps/findutils-4.2.33/patches/force-getcwd-fallback.patch

@@ -5,8 +5,8 @@ SPDX-License-Identifier: GPL-3.0-or-later
 Use the fallback implementations of getcwd to get the same /usr/bin/find
 checksum when building across FUSE and non-FUSE filesystems.
 
---- gnulib/m4/getcwd-path-max.m4	2022-01-08 13:16:54.412709192 +0200
-+++ gnulib/m4/getcwd-path-max.m4	2022-01-16 11:38:02.658606802 +0200
+--- findutils-4.2.33/gnulib/m4/getcwd-path-max.m4	2022-01-08 13:16:54.412709192 +0200
++++ findutils-4.2.33/gnulib/m4/getcwd-path-max.m4	2022-01-16 11:38:02.658606802 +0200
 @@ -81,7 +81,7 @@
    char *cwd = getcwd (buf, PATH_MAX);
    size_t initial_cwd_len;