Do use readlink if available

Do use readlink if available

The main effect of this patch is to make gnulib-tool not spam the
terminal with failures from ls. Despite the copious stderr output,
files still get linked correctly.

	gnulib-tool: Use readlink if it is available.
	* gnulib-tool (func_readlink): Choose function more appropriately.

Running under dash, type -p readlink fails because dash doesn't
understand -p. That causes gnulib-tool to fall back to ls to read
symlinks, despite readlink being available. That, in turn, spams the
terminal when func_ln_if_changed's DEST argument doesn't exist.

The output from type goes to /dev/null anyway, so asking for -p has no
purpose.