scripts: fix sysroot prefix dir

The sysroot prefix dir was broken in #4960f5d9f829 due to a mishap
when making the out-of-sysroot lib/ symlink: the './' was mistakenly
changed into a single '.' .

Although Jonathan suggested restoring the missing '/' to restore it to
normal operation, I prefered using an explicit pushd/popd to be extra
sure of the symlink location and target, along with a fix in the sysroot
relative directory calculation.

Reported-by: Jonathan Grundon <JGrundon@xos.com>
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@anciens.enib.fr>

diff -dur strace-4.5.15.orig/signal.c strace-4.5.15/signal.c

--- strace-4.5.15.orig/signal.c	2007-01-11 23:08:38.000000000 +0100

+++ strace-4.5.15/signal.c	2007-07-14 19:24:43.000000000 +0200

@@ -1440,7 +1440,7 @@

 	  	tcp->u_rval = tcp->u_error = 0;

 		if(tcp->u_arg[0] == 0)

 		  	return 0;

-		tcp->auxstr = sprintsigmask("mask now ", tcp->u_arg[1]);

+		tcp->auxstr = sprintsigmask("mask now ", tcp->u_arg[1], 0);

 		return RVAL_NONE | RVAL_STR;

 	}

 	return 0;