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/dhcpcd-10.0.1/patches/remove-ctassert.patch

@@ -5,8 +5,8 @@
 __CTASSERT macro does not work properly on our older GCC.
 
 diff -ru src/arp.c src/arp.c
---- src/arp.c
-+++ src/arp.c
+--- dhcpcd-10.0.1/src/arp.c
++++ dhcpcd-10.0.1/src/arp.c
 @@ -63,9 +63,6 @@
  /* ARP debugging can be quite noisy. Enable this for more noise! */
  //#define	ARP_DEBUG
@@ -18,8 +18,8 @@ diff -ru src/arp.c src/arp.c
  arp_request(const struct arp_state *astate,
      const struct in_addr *sip)
 diff -ru src/auth.c src/auth.c
---- src/auth.c
-+++ src/auth.c
+--- dhcpcd-10.0.1/src/auth.c
++++ dhcpcd-10.0.1/src/auth.c
 @@ -343,9 +343,6 @@
  
  	/* RFC3318, section 5.2 - zero giaddr and hops */
@@ -31,8 +31,8 @@ diff -ru src/auth.c src/auth.c
  		memset(mm + offsetof(struct bootp, giaddr), 0, 4);
  	}
 diff -ru src/dhcp6.c src/dhcp6.c
---- src/dhcp6.c
-+++ src/dhcp6.c
+--- dhcpcd-10.0.1/src/dhcp6.c
++++ dhcpcd-10.0.1/src/dhcp6.c
 @@ -84,33 +84,28 @@
  	uint8_t xid[3];
  	/* followed by options */
@@ -76,8 +76,8 @@ diff -ru src/dhcp6.c src/dhcp6.c
  struct dhcp6_op {
  	uint16_t type;
 diff -ru src/dhcp.c src/dhcp.c
---- src/dhcp.c
-+++ src/dhcp.c
+--- dhcpcd-10.0.1/src/dhcp.c
++++ dhcpcd-10.0.1/src/dhcp.c
 @@ -98,11 +98,6 @@
  #define IP_RECVPKTINFO IP_PKTINFO
  #endif
@@ -91,8 +91,8 @@ diff -ru src/dhcp.c src/dhcp.c
  	uint8_t value;
  	const char *name;
 diff -ru src/if-bsd.c src/if-bsd.c
---- src/if-bsd.c
-+++ src/if-bsd.c
+--- dhcpcd-10.0.1/src/if-bsd.c
++++ dhcpcd-10.0.1/src/if-bsd.c
 @@ -1600,7 +1600,6 @@
  #endif
  }
@@ -102,8 +102,8 @@ diff -ru src/if-bsd.c src/if-bsd.c
  if_handlelink(struct dhcpcd_ctx *ctx)
  {
 diff -ru src/ipv6nd.c src/ipv6nd.c
---- src/ipv6nd.c
-+++ src/ipv6nd.c
+--- dhcpcd-10.0.1/src/ipv6nd.c
++++ dhcpcd-10.0.1/src/ipv6nd.c
 @@ -80,7 +80,6 @@
  	uint32_t	nd_opt_rdnss_lifetime;
          /* followed by list of IP prefixes */
@@ -121,8 +121,8 @@ diff -ru src/ipv6nd.c src/ipv6nd.c
  
  /* Impossible options, so we can easily add extras */
 diff -ru src/privsep-root.c src/privsep-root.c
---- src/privsep-root.c
-+++ src/privsep-root.c
+--- dhcpcd-10.0.1/src/privsep-root.c
++++ dhcpcd-10.0.1/src/privsep-root.c
 @@ -56,8 +56,6 @@
  #include "sa.h"
  #include "script.h"