sys/base/readlink: bugfix involving resolution of symlinks in $PWD

Symlinks in the present working directory were not always resolved
when canonicalising a path using -f/-e/-m; if a relative path was
given that contained only one directory component (or only has an
initial ./), then readlink failed to use 'chdir' to resolve symlinks
in $PWD (the present working directory).

lib/modernish/mdl/sys/base/readlink.mm:
- Bugfix. Generally, '.' path components must be skipped so that
  'readlink -m' works correctly. This erroneously skipped an
  (implicit or given) initial './' so the PWD was not
  canonicalised. So add a line to special-case that.
      In that case, also make sure $PWD exists, and refuse to
  canonicalise (even with -m) if not; this behaviour matches GNU
  'readlink'.

lib/modernish/tst/sys.t:
- Add regression test that catches this bug.

lib/modernish/mdl/sys/base/readlink.mm

@@ -100,6 +100,7 @@
 	# Canonicalise the path using 'chdir' to convert to physical path.
 	# If -m given, emulate traversal of nonexistent paths.
 	str in "$1" '/' || set -- "./$1"
+	str begin "$1" './' && { str ne "${PWD:-.}" '.' && is -L dir "$PWD" && chdir -f -- "$PWD" || \exit 0; }
 	{ str end "$1" '/.' || str end "$1" '/..'; } && set -- "$1/"
 	unset -v _Msh_nonexist
 	IFS='/'

lib/modernish/tst/sys.t

@@ -49,6 +49,19 @@ TEST title="readlink: perms enforced whl traversing?"
 	return 1
 ENDT
 
+TEST title='readlink: symlinks in $PWD are resolved'
+	# chdir into a symlink to a directory, canonicalise a path within it, check the symlink in $PWD is resolved.
+	v=$(
+		umask 077
+		mkdir -p $tempdir/sym/d1/d2
+		ln -s d1 $tempdir/sym/sym2dir
+		chdir -fL $tempdir/sym/sym2dir || exit 1
+		readlink -f d2
+	) || return 1
+	failmsg=$v
+	str eq $v $tempdir/sym/d1/d2
+ENDT
+
 # ... sys/base/tac ...
 
 TEST title='tac: default'