Add -W/--writable function to checkpath

Checkpath -W will use access(3p) to determine whether or not a path is
writable. This is more accurate than test(1p) because it also takes into
account whether or not the filesystem is mounted read-only.

Modified by William Hubbs to add the man page update.

man/runscript.8

@@ -315,6 +315,11 @@ Mark the service as inactive.
 .Xc
 Checks to see if the path exists, is of the right type, owned by the right
 people and has the correct access modes. If not, then it corrects the path.
+.It Ic checkpath
+.Op Fl W , -writable
+.Ar path
+.Xc
+checks to see if the path is writable.
 .It Ic yesno Ar value
 If
 .Ar value